Unroll the pie crust and precisely fit it in a tart pan (11 inches). Poke the dough all over with a fork. Then put the pie in the fridge.
Preheat oven at 390F.
On a cutting board, sliced the tomatoes (⅛ inches) and put them on a plate. Salt them lightly and set them aside. This step will help to remove some juice from the tomatoes so the crust will not be too soggy (*).
In a skillet, on medium-low heat, add 1 tablespoon olive oil, add the onions with salt, pepper, herbes de Provence, and let them cook until soft. Add a splash (about 1-2 Tbsp) of balsamic vinegar and sugar. Cook until the onions absorb all the vinegar and there is no juice. Set aside.
Take your tart pan from the fridge. Spread evenly Dijon Mustard all over the bottom in a fine layer. Sprinkle evenly grated cheese on top. Scatter ¾ of the onions on the cheese. Take each tomato slice by shaking them gently before placing them nicely on the tart.
Add the ¼ left onions on the top and sprinkle more grated cheese if you want. Drizzle some olive oil.
Cook for 25 - 30 minutes until the crust and the cheese are lightly golden.
Enjoy this delicious tart with a green salad on the side!
Notes
(*) You can always add a few breadcrumbs or rolled oats before putting the tomatoes if you are afraid of the sogginess of the dough. It is good too!
Keyword dijon mustard, how to make tomato mustard tart, savory tart, summer, tomato