8ounceslasagna noodlesabout 1/2 pound, broken into 3 - 4 inch pieces
3cupsshredded mozzarella cheeseabout 12 ounces
15ouncespart-skim ricotta cheesewhole milk ricotta works fine too
1/4cupshredded Parmesan cheese
12-inchor larger cast-iron skilletor other oven-proof skillet
Heat oven to 375 degrees. Place a 12-inch or larger cast-iron skillet on the stovetop over medium heat. Heat olive oil and add onions, stirring frequently, until they begin to soften. Add the garlic, sauteeing for another minute. Add the zucchini and mushrooms and saute for 3 - 4 minutes until the zucchini is tender and the mushrooms soften.
Pour Italian tomato sauce over veggies and combine, stirring over medium heat until warmed through. Stir in baby spinach leaves and cook for an additional minute. Taste sauce and add a little salt and pepper if desired.
Remove pan from heat.
Remove all but 1 cup of the sauce from the pan. Spread the remaining sauce evenly in the bottom of the pan and cover with a layer of noodles. No need to place them perfectly evenly, just place them here and there in a single layer until most of the sauce is covered by the pasta.
Spoon the ricotta, about 5 or 6 mounds and about 1/3 of ricotta, onto the noodles. Add 1/3 of the remaining sauce, spreading evenly with the back of a spoon, and then sprinkle 1/3 of the mozzarella. Add another layer of noodles. Repeat with another layer - ricotta, sauce, mozzarella, and noodles, then spoon the remaining sauce on the top, add the ricotta, sprinkle with the remaining mozzarella cheese, and top with the Parmesan cheese.
Place the skillet in the oven and bake for about 30 minutes, until the cheese is bubbly and the pasta is cooked through.
Remove from oven and allow to cool for about 5 minutes, then serve.