Vegetarian Skillet Stuffed Shells

Name: Vegetarian Skillet Stuffed Shells
Ingredients: 18 jumbo pasta shells (about 6 oz.); 1 1/2 tsp. kosher salt, divided, plus more; 2 Tbsp. extra-virgin olive oil; 1/2 lb. crimini mushrooms, thinly sliced; 1 tsp. freshly ground black pepper; 1/2 cup dry white wine or vermouth; 5 oz. baby spinach; 6 garlic cloves, thinly sliced; 2 Tbsp. unsalted butter; 3 cups marinara sauce; 1/2 tsp. crushed red pepper flakes; 2 cups whole-milk ricotta; 3 oz. finely grated Parmesan (about 1 cup), plus more for serving; 3 Tbsp. finely chopped oregano, divided