In the morning, place the ricotta in a strainer over a bowl, or wrapped in cheese cloth in a bowl, and leave it in the fridge for the day. This will drain excess water from the ricotta and prevent a watery lasagna when cooking. If you don't have time to do this, or if you forget, don't fret—1/2 cup of ricotta won't add too much extra water to your lasagna. I just prefer to get that extra water out.
Preheat the oven to 375º F.
Place the skillet over medium-high heat on the stove and add the olive oil. Once the oil is shimmering, add the mushrooms along with a pinch of salt and pepper. Sauté the mushrooms until they are golden and tender—about 3-5 minutes.
Add the spinach and the drained diced tomatoes. Stir and sauté for another minute. Finally, add the marinara and the broken lasagna noodles. Carefully stir the noodles into the sauce and ensure that all of the noodles are covered in sauce so that they cook properly.
Spoon small dollops of ricotta over top of the sauce and noodles, then sprinkle the top with the shredded cheese.
Cover the skillet with tin foil and bake for 20 minutes, or as per the instructions on the oven-ready noodle package. After 20 minutes, remove the foil, and turn on the broiler. Broil the lasagna for 3-5 minutes or until the cheese is bubbly and starting to turn a nice golden hue. Keep a close eye on it as it broils, since all broilers are different and time will vary.
When done, remove the skillet from the oven and place on a heat-resistant surface to set for 5-10 minutes. Garnish with fresh basil leaves, and dig in.