Preheat oven to 375°F. Cook and drain manicotti shells according to package directions. Keep shells in cold water until ready to use.

While pasta is cooking, heat olive oil over medium heat in a large skillet. Add in the chopped onion, cooking for 3 - 5 minutes. Add the garlic and cook for about one minute. 

Add in the sliced mushrooms and saute for 3 - 5 minutes more. Finally, add the fresh spinach and allow to wilt for about 1 minute. Remove from heat and cool.

In a small bowl, mix together the ricotta and Parmesan cheese, add to the slightly cooled mushroom mixture, and season with salt and pepper to taste. 

Pour half of the Campbell’s® Condensed Cream of Mushroom Soup (that has already been mixed with cream or milk) into the bottom of an 8-inch baking dish.

Using a small spoon, gently stuff each manicotti shell with 2 - 3 tablespoons of mushroom ricotta mixture and place in a single layer over the Campbell’s® Condensed Cream of Mushroom Soup in the dish.

Pour the remaining Campbell’s® Condensed Cream of Mushroom Soup over the top of the pasta shells, garnish with grated Parmesan and fresh thyme if desired, and bake in a 375°F preheated oven for about 15 minutes or until bubbly.