Place mushrooms, 1 tablespoon olive oil, and 1/2 teaspoon dried oregano in a microwave-safe bowl. Heat on high for approximately 5 minutes to release some of the moisture from the mushrooms. Pour out any accumulated moisture from the bowl. Set mushrooms aside.
In a 12-inch non-stick skillet, drizzle 2 tablespoons oil. Spread about 1/2 cup of spaghetti sauce over the bottom of the skillet. Sprinkle about 2 tablespoons Parmesan cheese over the sauce.
Layer enough of the ravioli to almost cover the sauce layer, approximately 10 or 11.
Sprinkle grated mozzarella over the ravioli, to your liking, covering all the ravioli pieces and depending on how cheesy you want your lasagna.
Place about 1/3 of the mushrooms over the mozzarella cheese.
Sprinkle about 1 tablespoon of the fresh basil over the mushrooms and mozzarella.
Add half of the chopped spinach, spreading over all.
Cover this layer with more sauce to thinly cover. Cook's note: As you can tell, you can't get this wrong. Add the quantites of most of the ingredients to your liking as you go.
Sprinkle a generous amount of Parmesan cheese over the sauce. Adding the Parmesan to the sauce layer helps to thicken it as it cooks.
Now repeat with another layer of ravioli, mushrooms, mozzarella, basil, spinach, sauce, and Parmesan.
For the final layer (top), place only 3 or 4 more raviolis over the layers, more sauce, then more Parmesan, and mozzarella. Add the rest of the mushrooms and sprinkle more basil, using all or some of what you have left, to your liking.
Now sprinkle Parmesan once more over everything and some freshly grated black pepper.
Turn heat to medium until the sauce starts barely bubbling through the ravioli. Turn to medium-low and simmer uncovered for 20 minutes.
Cover for cook until mozzarella topping has melted and lasagna is nice and bubbly, approximately 5 minutes.
Uncover and try not to let any moisture that accumulated on the lid drip back into the dish. Let sit for 10 minutes before serving.
Leftovers are even better, as seems to be the case with most casserole type recipes.
If you want to add meat to this recipe, brown one pound ground beef in the skillet (or half beef and half Italian sausage) until no longer pink on the outside. Drain and then remove the meat to a plate. Using the same skillet, start your layers adding the meat into the sauce layer during assembly.