In a large skillet, heat 1 tablespoon olive oil over medium heat. Add the onion and cook, stirring, until softened, about 8 minutes; push the onions to the side and add the sausages. Cook, breaking up the meat, until browned, about 5 minutes. Stir in the mushrooms and remaining 1 tablespoon olive oil; cook until softened, about 5 minutes. Stir in the parsley, remove from the heat and let cool for 10 minutes. Stir in the ricotta. Season with salt and pepper.
Preheat the oven to 375 degrees . Lay 1 phyllo sheet on a work surface and cover the remaining sheets with a damp kitchen towel. Brush the sheet with butter, then set another sheet on top. With a sharp knife, trim the phyllo to make a square. Brush with butter, place a quarter of the filling in the center and spread into a 4-inch square. Fold the flaps of phyllo over the filling to form a pocket; brush with butter; repeat with the remaining ingredients to make 3 more pockets.
Brush a baking sheet with butter and place the pockets on top. Bake until golden, 25 to 30 minutes.