Preheat oven to 375 F. Combine water and vinegar in a small bowl. Combine flour and salt in another bowl. Using a pastry cutter or your fingers, cut butter and cream cheese into flour mixture until mixture resembles coarse crumbs with some larger pieces remaining.
Add water mixture to dough in a slow, steady stream, stirring, until mixture just begins to hold together. (Alternatively, pulse ingredients in a food processor.) Turn out onto a piece of plastic wrap. Press dough into a disk. Refrigerate until firm, about 1 hour or overnight.
Add olive oil to frying pan on medium-high heat. Add the onion and cook until soft and lightly colored (about 7 minutes). Be sure to stir the onions occasionally. Add thyme, salt, and some crushed pepper. Towards end of the cooking, add in the tuna and spinach. Cook for about another 2 minutes.
While this cooks, in a medium sized bowl whisk eggs with cream and milk. Add a little salt and pepper. Stir in the cooled tuna mixture.
Roll dough to fit your chosen tart pan, then drape dough in pan. Neatly press dough up the sides of the pan and shape it. Set it on a sheet pan. Pour in the egg mixture. Place the tart pan over a foil lined cookie sheet.
Bake for about 35-40 minutes or until the tart is golden brown and the filling is set. Check tart at about 30 minutes. If the tart shell is browning too much, cover the tart with foil paper for the rest of the baking time. Let tart cool before removing from tart pan or slice directly in the tart pan when warm.