Red Onion Tart with Chestnuts and Blue Cheese

While I bake tasty onion tarts quite often, the one I have here turned out even tastier than usual. What made it so very tasty lies in the addition of roasted chestnuts to the dough, and flavoring the filling with a bit of mustard, along with blue cheese and rosemary. This combination of flavors and textures took the plain onion tart into another level. Try it and enjoy.

Notes:
* If you can’t find fresh chestnuts, ½ cup vacuum packed peeled chestnuts can be used instead, though the texture will be a bit different.
* If you don’t have a rectangular tart pan, a large round tart pan, or 6 small ones, can be used instead.
* As you can see from the photos, I ended up with extra dough and filling, so I baked a small tart with it. Wrapped and frozen, it can be used for a quick lunch anytime.
* For a simpler onion tart recipe, check THIS recipe.

Makes: 10
Prep time: 30 minutes
Cooking time: 20 minutes
Blind baking time: 15 minutes
Baking time: 35 minutes for the small tart; 45 for the large

Ingredients:
For the dough:
About 20 medium chestnuts (see notes)
1 cup white flour
1 cup spelt flour
1 tsp salt
1 stick (115 grams) butter, cold, cut into cubes
1-2 Tbs ice water
For the filling:
1 Tbs oil
2 Tbs butter
1½ medium red onions, sliced
2 tsp salt
½ tsp freshly ground 4 pepper mix
2 tsp brown sugar
1 Tbs chopped rosemary
1 Tbs Balsamic vinegar
¾ cup sour cream
2 XL eggs
1 tsp Dijon mustard
½ tsp mustard powder
½ cup crumbled blue cheese
For brushing the dough:
1 Tbs Dijon mustard
For serving (optional):
Fresh endive or micro greens with honey-mustard dressing (recipe HERE)

1. The chestnuts: with a sharp knife, cut an X at the top of each chestnut. Roast in a hot toaster oven for about 15 minutes, until they burst open. Let cool a bit and peel. Process coarsely in a food processor, fitted with the metal blade.
2. The dough: add the white and spelt flour, salt and butter to the bowl with the chestnuts. Process to a crumbly mixture. Add 1 Tbs ice water and process to form dough. Add the remaining 1 Tbs water, only if needed. The less you’ll handle the dough, the flakier it will be after baking. Wrap the dough with wax paper and keep in the fridge for an hour.

3. The filling: in a large pan, warm the oil and butter over medium-high heat. Add the onions, salt, pepper, rosemary and Balsamic vinegar. Fry, mixing occasionally, for about 20 minutes, until the onions soften and caramelize and most of the liquids absorbed. Transfer to a large bowl and let cool to room temperature. Add the sour cream, eggs, mustard and mustard powder and mix well. Taste and adjust seasoning.

4. Preheat the oven to 340F (170C). Roll the dough to a rectangular a bit larger than the 7.5”x 4.25” (12×36 cm) tart pan. Place the dough in the pan and press it to the bottom and sides. Cut off extra dough (I’ve used the cut-off dough to bake a small 4” (10cm) tart). Prick the dough with a fork in several places, cover with baking paper and add some beans on top, to prevent the dough from rising while baking. Bake for 15 minutes. Take the pans out of the oven, but keep it on. Remove the baking paper and the beans and let cool a bit.

5.  Brush the dough with the mustard and scatter half of crumbled blue cheese over it. Add the filling and scatter the rest of the blue cheese on top.

6. Bake until set and golden, about 35 minutes for the small tart, and 45 for the large one. Transfer to a rack to cool a bit before cutting and serving.