Sylvie, I made your tarte aux thon (tuna quiche) last night. This is another favorite in our household.
I tend to forget to do this, but am trying to remember more and more because cleaning out the dish can be a pain in the tush. First, line the dish with pastry paper (or whatever it's called). Then lay out the pre-made pastry crust in the dish. I used the pur beurre crust this time. Cook in the oven for about 5 or so minutes. While the pastry's cooking, brown an chopped onion in a pan with olive oil. Add a can of crushed tomatoes. Add a can of saltless tuna. Cook this mixture in order to reduce the liquid.
After you pull out the pastry crust from the oven, coat the bottom of the crust with some mustard. Next, pour the tuna-onion-tomato mixture into the crust. Top that with little dollops of creme fraiche. Cover that with some shredded Gruyere cheese. And then you can add some sliced tomatoes on the top.
Cook until it's a nice color. I usually cook it for 15 - 20 minutes.