Step 2
To prepare onion filling: Meanwhile, heat 3 tablespoons oil in a large skillet over medium heat. Add onions and cook, stirring frequently, until golden brown and very soft, about 25 minutes. (If they begin to stick, add water ¼ cup at a time to release them and prevent burning.) Add mushrooms and cook, stirring, until just beginning to soften, about 3 minutes. Add wine and 1 teaspoon salt and continue cooking until most of the liquid is absorbed, 2 to 3 minutes. Remove from heat; stir in pepper.