Weeknight Sausage Pasta with Spinach

Name: Weeknight Sausage Pasta with Spinach
Prep time: 10 min
Cook time: 25 min
Total time: 35 min
Cuisine: European, American, Italian, Mediterranean
Ingredients: 1 small yellow onion; 3 cloves garlic; 1 pound uncooked hot or sweet Italian sausage; 2 tablespoons olive oil; 1/4 teaspoon red pepper flakes (optional); 1 (28-ounce) can crushed tomatoes; 1/2 teaspoon kosher salt, plus more for the pasta water; 1/4 teaspoon freshly ground black pepper; 1 pound dried short pasta, such as penne; 3 ounces baby spinach (about 3 packed cups); Grated Parmesan cheese, for serving