Heat the oil in a large skillet. Add the garlic, onion, and mushrooms; sauté until soft. Add the spinach stems and ribs and toss until they begin to soften.
Add the spinach leaves, salt, and pepper, and ¼ cup stock, and keep tossing until the mixture is combined and the stock is absorbed or evaporated. If the mixture is very thick, add more water. Fold in the cream and mozzarella cheese and stir until there is a uniform mixture. Taste and adjust salt and pepper.
Place a wonton wrapper on a cutting board. Put about 1 teaspoon of the filling in one corner of the wonton wrapper, leaving about ½ inch margin around the edges. Fold the wrapper into a triangle, pressing the filling so that it is evenly distributed. Using the tines of a fork, press down along all the edges to seal. Continue with the other eleven wrappers.
To cook, you can boil, fry, or bake the ravioli.
Boil: drop into a pot of boiling water and cook for 5 minutes (this sometimes leads the ravioli to open, which is a big mess).
Fry: Heat oil in a heavy skillet and fry on one side, then turn once to fry the other side.
Bake: Place on a greased cookie sheet and bake in a 325 F oven for 20 minutes, then flip and bake for another 10 minutes.