Caramelized Onion Tart

Ingredients

• 135 g (1 cup + 1 tbsp)gluten-free flour
• 50 g (½ cup)almond flour
• 2 tbspground flax seeds or ground chia seeds
• 75 ml (5 tbsp)water
• 35 g (2½ tbsp), softenedcoconut oil
• ⅓ tspsea salt
• 3 medium-largeonions, peeled and diced
• 2 mediumbell peppers, diced
• 2 small stalksleek, finely chopped
• 3 clovesgarlic, minced
• 3 tbspchickpea flour
• 1 heaped tbspcornstarch or arrowroot flour
• 80 ml (⅓ cup)water or vegetable broth
• 1 tsponion powder
• ¾ tsp, or more or less to tastesea salt
• ½ tsppaprika
• ½ tsp, optionalcumin
• to tasteground black pepper
• ½ tbsp fresh or ½ tsp driedthyme
• 1 tbsptamari, soy sauce (gluten-free if needed) or coconut aminos
• 180 ml (¾ cup)plant-based milk
• 198 g (7 oz), optionalvegan cheese of choice
• to taste, optionalfried onion rings for topping

Instructions

1. 1

   Soak the flax

   Whisk the ground flax seeds or chia seeds with 75 ml (5 tbsp) water in a small bowl. Leave to stand for 5 minutes.

2. 2

   Mix the crust

   Put the gluten-free flour, almond flour, soaked flax mixture, softened coconut oil and ⅓ tsp sea salt in a bowl. Mix with a spatula or your hands until the dough holds together; you can also use a food processor.

3. 3

   Chill the crust

   Press the dough into a greased or oiled 20 cm (8-inch) tart or pie dish, bringing it up the sides. Place the dish in the fridge.

4. 4

   Mix the thickener

   Whisk the chickpea flour, cornstarch or arrowroot, onion powder, ¾ tsp sea salt, paprika, optional cumin, black pepper and 80 ml (⅓ cup) water or vegetable broth in a bowl. Set aside.

5. 5

   Cook the vegetables

   Heat oil in a pan over low to medium heat, then add the onions, leek and bell peppers. Cover and cook for about 15–20 minutes, stirring frequently, until lightly caramelised. Stir in the garlic for the last few minutes, and preheat the oven to 200°C (390°F).

6. 6

   Thicken the filling

   Add the chickpea flour mixture to the pan and stir straight away, as it thickens quickly. Stir in the plant-based milk, tamari and thyme.

7. 7

   Fill the crust

   Let the filling simmer for a few more minutes, until considerably thicker, then turn off the heat. Spoon it into the chilled crust and smooth it evenly. If using, add vegan cheese and pan-fried onion rings on top.

8. 8

   Bake the tart

   Bake for about 25–30 minutes, until nicely browned.

What is Zwiebelkuchen?

Zwiebelkuchen is a German savoury onion tart associated with autumn. This version takes inspiration from that comforting style, using a gluten-free crust and a plant-based filling.

The onions, leek and peppers provide the savoury base, while chickpea flour and starch help the filling set. Almond flour and flax give the crust structure without wheat or eggs.

Nutrition and macros by portion

The nutrition label shows one serving as written: 216 calories, 5g protein, 26g carbs, 9g fat.

The crust and vegetables contribute most of the carbohydrates, while coconut oil and optional vegan cheese contribute fat. Chickpea flour, almond flour and the cheese add protein; serve a slice with a generous salad or vegetables to round out the plate.

Building a reliable crust

Flax binder: soaking the ground seeds for 5 minutes gives them time to thicken, helping the dough hold together.

Even pressing: push the dough across the base and up the sides of the dish with an even thickness, so it bakes consistently. Chilling the shaped crust while you make the filling helps it firm up.

Soft, sweet vegetables

Low heat: cook the onions, leek and peppers covered for 15–20 minutes, stirring regularly. The lid helps the vegetables soften before they pick up a little colour.

Garlic timing: add the garlic only for the final few minutes of cooking, as it can become bitter if left over the heat too long.

Getting the filling to set

Thickener: whisk the chickpea flour and starch with the liquid and seasonings before adding it to the pan. Stir it in promptly because the mixture thickens quickly.

Doneness cue: simmer until the filling has thickened considerably before putting it in the crust. This helps it stay in place when the tart is cut.

Baking and serving

Colour: bake at 200°C for 25–30 minutes, looking for a nicely browned top. The filling is already cooked, so the oven time is mainly for baking the crust and bringing the tart together.

On the plate: serve warm or at room temperature with a crisp green salad or steamed vegetables. The optional vegan cheese and fried onion rings make the topping richer and more substantial.

Ingredient choices

Flour options: use the gluten-free flour and almond flour as listed for the intended crust texture. The recipe also allows chia seeds in place of flax, and arrowroot in place of cornstarch.

Seasoning: tamari brings savoury depth, while thyme, paprika and optional cumin add warmth. Choose gluten-free tamari if needed, or use coconut aminos as an alternative.

Recipe notes & tips

• Keep the vegetable heat low to medium so the onions soften without scorching.
• Stir the vegetables frequently while they cook under the lid.
• Prepare the chickpea flour mixture before adding it to the hot pan so you can stir straight away.
• Press the crust evenly into the corners and sides of the dish.
• Check the tart at 25 minutes and bake longer if it has not browned to your liking.

Variations

• Use chia: swap the flax seeds for ground chia seeds in the crust.
• Change the starch: use arrowroot flour instead of cornstarch.
• Add vegan cheese: scatter your preferred vegan cheese over the filling before baking.
• Top with onions: add pan-fried onion rings before baking for extra onion flavour.
• Adjust the seasoning: leave out the cumin or add more black pepper to suit your taste.

Common mistakes to avoid

• Skipping the flax soak: the seeds need 5 minutes in the water to thicken; let them stand before mixing the dough.
• Rushing the vegetables: cook them for the full 15–20 minutes, stirring often, so they soften and lightly caramelise.
• Adding the thickener slowly: pour in the prepared mixture and stir immediately because it thickens fast.
• Using a runny filling: simmer until it has thickened considerably before adding it to the crust.
• Baking by time alone: use the 25–30 minutes as a guide and look for a nicely browned tart.

How to store

Cover leftover tart and refrigerate for up to 3–4 days. Reheat in the oven until warmed through, or serve it at room temperature.

Frequently asked questions