Cook shells in boiling salted water for 9 minutes; drain and run under cold water.
STEP 2
Heat oil in a skillet over high heat. Add mushrooms and cook until browned, 5-6 minutes. Season with black pepper and 1 tsp salt. Add wine and cook until reduced by half, 1-2 minutes. Stir in spinach and cook covered until wilted, about 3-5 minutes. Transfer mixture to a bowl.
STEP 3
In the same skillet, cook garlic and butter over medium-high until garlic is fragrant, about 3 minutes. Add marinara sauce and crushed red pepper; simmer for 6-8 minutes.
STEP 4
In the bowl, mix ricotta, 3 oz Parmesan, 2 Tbsp oregano, and the remaining salt. Fill each shell with about 2 Tbsp of the mixture.
STEP 5
Nestle filled shells into the sauce in the skillet, cover, and cook until heated through, 4-6 minutes. Let sit for 5 minutes, then sprinkle with Parmesan and remaining oregano.