Spinach Stuffed Shells

Name: Spinach Stuffed Shells
Prep time: 35 min
Cook time: 36 min
Total time: 1 h 11 min
Cuisine: European, Italian
Ingredients: 10 ounces baby spinach (about 10 cups), or 1 (10-ounce) package frozen chopped spinach; 3 cloves garlic; 12 ounces low-moisture whole-milk or part-skim mozzarella cheese, shredded (about 3 cups); 1 ounce Parmesan cheese, finely grated (about 1/2 firmly packed cup grated on a Microplane or 1/3 cup store-bought); 3 tablespoons olive oil, divided; 1/4 teaspoon red pepper flakes; 3/4 teaspoon kosher salt, divided, plus more for the pasta water; 6 ounces dried jumbo pasta shells (about 20); 1 (15- to 16-ounce) container whole-milk ricotta cheese (about 2 heaping cups); 1/4 teaspoon freshly ground black pepper; 1 (about 24-ounce) jar marinara sauce (3 cups), divided