Bring a large pot of lightly salted water to a boil; add pasta and stir. Bring back to a boil, cover and remove from heat; let stand 10 min. Drain and rinse pasta; set aside.
In a large nonstick skillet over medium-high heat, cook sausage, onion and mushrooms, breaking up sausage, until onion is tender, 8 to 10 min; set aside.
In a bowl, combine ricotta, egg, ¼ tsp salt, ½ cup cheese blend and spinach. Stir in cooked sausage mixture; set aside.
In same skillet, cook eggplant, stirring until just softened, about 8 min. Add pasta sauce and remaining salt and bring to a boil.
Heat oven to 375°F. Spoon ⅔ of the eggplant sauce into the bottom of a shallow 2½-quart casserole dish. Fill each shell with about 2 tbsp sausage mixture, arrange in dish and spoon remaining sauce over shells. (Wrap and freeze at this point if desired.)
Cover and bake until sauce is bubbling, about 25 min. Remove from oven and sprinkle shells with 1¼ cups cheese blend; bake, uncovered, until cheese has melted, 8 to 10 min more.
This recipe makes a large quantity; to freeze half for a later meal, split prepared shells and sauce between two smaller casserole dishes. Wrap one casserole dish well in aluminum foil and freeze for up to 2 months. To serve, thaw overnight in the refrigerator and bake 15 min more than directed.