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.
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. Prick bottom of the tart with a fork.
In a medium bowl, whisk together the ricotta and egg. Stir in the steamed spinach, peas, Parmigiano Reggiano, mint, chives, ½ teaspoon salt, and 1 teaspoon black pepper.
Spoon filling into tart shell and spread around evenly.
Dot top of tart with butter.
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. If you remember, you could place a few mint sprigs on top before serving.