3poundswhite onion[1.4 kilogram], halved and sliced thinly
½teaspoonfresh thyme leavesor dry if you don't have fresh
1tablespoongarlic salt(see notes)
1teaspoonblack pepperfreshly-cracked, or ground
1cupgrated Gruyère cheese
½cupshredded sharp cheddar
For the crust
In a mixing bowl, mix flour, salt, and baking soda. Pour the mix into the food processor.Pour in melted butter and egg, and pulse to incorporate. Midway stop and scrape the sides with a spatula moving the flour from the sides to the middle. If the dough is too dry, add cold water by the tablespoon until the dough just forms a ball but is not soggy or sticky.
Pour onto the counter and knead the dough lightly until it is smooth (about 1 minute).Wrap in plastic film and let it rest at room temperature for at least 10 minutes.
For the filling.
Heat the oil over medium-low heat in the skillet. Add the onions and thyme and stir until the onions are coated with oil. Cover and simmer over for 5 minutes, stirring often.
Add the sherry, and simmer until the sherry has mostly evaporated. Cover and simmer over low heat for another 25-30 minutes, or until the onions have turned caramel brown and are very tender.Season with garlic salt, pepper, and nutmeg.
Increase heat to medium-high heat and add flour, sprinkling, and mixing until all the onions are lightly coated with flour. Remove from the heat.Let it rest until the onion mixture has cooled to room temperature.
Heat oven to 350 ºF [175 ºC].
Roll out the dough with a rolling pin until you have a circle big enough to cover the bottom and sides of the tart pan.Cover the tart pan with the dough and tidy up the sides using any excess dough. Poke a few holes in the bottom with a fork. Line the bottom with a circle of parchment paper, and place the pie weights or dry beans on the parchment paper.
Bake the pie crust in the preheated oven for 15 minutes. Remove from the oven.Fill the pie crust with the onion mixture, and top with the cheeses.
Return to the oven and bake until the cheese is bubbly and golden brown.Remove from the oven and serve hot.
If you don't have garlic salt mix ½ tablespoon of regular with ½ tablespoon of garlic powder.You can make this French onion tart with puff pastry for a different and entirely marvelous variation. Instead of making the crust as directed, just bake a circle of puff pastry (unweighed) on the bottom of the tart pan.Whole wheat dough is very fragile, and not at all elastic like regular dough, I've found that the best way to spread it is doing it between two plastic film pieces. There's going to be some patching too. Don't worry too much, that's why I called it "rustic".
Find full recipe at: Gruyere and Caramelized Onion Tart https://tiaclara.com/french-onion-tart-recipe/