One-Pot Vegetarian Spaghetti

Name: One-Pot Vegetarian Spaghetti
Prep time: 5 min
Cook time: 20 min
Total time: 25 min
Cuisine: Italian
Ingredients: 2 teaspoons extra-virgin olive oil; 1/2 medium yellow onion, chopped; 1 (8 oz.) package button mushrooms, sliced; 2 cloves garlic, minced; 1/2 teaspoon salt; 1/4 teaspoon black pepper; 1/4 teaspoon red pepper flakes (optional); 2 cups low-sodium vegetable broth; 1 (14.5 oz.) can fire-roasted diced tomatoes; 10 oz. thin spaghetti noodles (I use whole wheat); 3 cups fresh spinach or baby spinach; 1/4 cup grated Parmesan cheese, plus extra for serving (see notes); 1/4 cup fresh chopped basil or parsley (optional)