Preheat oven to 400° F.

In a large heavy pot, heat oil over medium heat and sauté onions for 3 to 4 minutes. Add mushrooms and increase the heat to medium-high. Cook for 3 to 4 minutes more or until onions are tender and most of the liquid from mushrooms has been absorbed.

Stir in spinach (it will look like a lot but will cook down considerably). Stir-fry until it is dark green and there is very little to no liquid left in the pan. Season with salt and pepper. Pour into a strainer and press to drain any remaining liquid.

Place a thin layer of sauce in a 9- by 13-inch baking dish and layer 1/3 of the ravioli on top in a single layer. Top with a thin layer of sauce, 1/2 of the mushroom-spinach filling and 1/3 of the cheese. Repeat this layer with 1/3 of the ravioli, a layer of sauce, the rest of the filling and 1/3 of the cheese. Top with remaining ravioli and sauce. Reserve the remaining cheese.

Cover the dish with a layer of parchment paper and then wrap in foil. Bake for 30 minutes. Remove the foil and parchment paper and top with remaining cheese. (You may garnish with thinly sliced red bell pepper and basil leaves if you like.) Place back in the oven and bake for 10 to 12 minutes more, until golden and bubbly.