Ricotta, Spinach, and Mushroom Lasagna

1 T. olive oil, plus more for greasing the skillet
8 ounces white mushrooms, sliced
1 clove garlic, cut in half and crushed
1 tsp. fresh thyme leaves, chopped, or ¼ tsp. dried thyme
Salt and freshly ground black pepper
1 (10-ounce) package frozen chopped spinach, thawed
1 C. ricotta
1 large egg
3 C. marinara sauce (Store-bought or homemade)
4 ounces no-boil lasagna noodles
2 T. freshly grated Parmesan cheese

Preheat the oven to 350°F with one rack in the middle. Preheat the skillet over medium heat for 2 minutes. Add the olive oil to the skillet, allow it to heat 1 minute, then add the mushrooms, garlic, thyme, ¼ tsp. salt, and ¼ tsp. pepper. Cook, stirring occasionally, until the mushrooms soften, about 10 minutes. Taste to check for seasoning and add salt and pepper as necessary. Remove and discard the garlic. Pour the mushrooms and any liquid into a bowl and set aside. In a medium-size bowl, combine the spinach with the ricotta, egg, ¼ tsp. salt, and ¼ tsp. pepper.

Rub a coating of olive oil on the skillet and then layer the ingredients in this order:

¾ C. of the ricotta mixture
1 C. of the marinara sauce
half of the mushrooms and their liquid
half of the noodles (break the noodles if necessary to cover the whole skillet)
¾ C. of the ricotta mixture
1 C. of the marinara sauce
the remaining mushrooms and any liquid
the remaining noodles
the remaining ricotta mixture
the remaining marinara sauce

Sprinkle the Parmesan evenly across the top of the lasagna. Cover the skillet tightly with aluminum foil and place in the oven. Bake until the mixture is bubbling and the lasagna noodles are tender, about 45 minutes. Remove the skillet from the oven and transfer it to a rack to cool slightly, about 10 minutes, before serving hot. Leftovers can be refrigerated in a covered container for up to 2 days.