How to Make It
Preheat oven to 400°F. Cook shells in boiling salted water according to package directions for al dente pasta. Drain.
Heat 1 tablespoon oil in a large skillet over medium. Add onion and garlic; cook, stirring often, until softened, about 5 minutes. Stir in tomatoes, 1½ teaspoons salt, and ¼ teaspoon pepper. Cook, stirring often, until slightly thickened, 6 to 8 minutes. Reserve ½ cup sauce. Spoon remaining sauce into a lightly oiled 13-by-9-inch baking dish.
Wipe skillet clean and heat 1 tablespoon oil over medium-high. Add mushrooms and remaining ¼ teaspoon salt and ½ teaspoon pepper; cook, stirring occasionally, until browned, about 5 minutes. Transfer to a bowl. Add remaining 1 tablespoon oil to skillet. Add spinach and cook, tossing, until wilted, about 1 minute.
Stuff shells with ricotta, mushrooms, and spinach and arrange in baking dish. Top with reserved cup ½ sauce and Parmesan. Bake until shells are heated through and cheese is melted, 12 to 15 minutes.