Garlicky Tortellini Soup With Sausage, Tomatoes, and Spinach

Ingredients (12 items)

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• 1 pound mild sausage, removed from casings
• 1 medium onion, finely chopped
• 6 cloves garlic, finely chopped
• 1 (32 ounce) carton chicken broth
• 1 (14 ounce) can chicken broth
• 1 pound frozen cheese tortellini
• 1 (14 ounce) can diced tomatoes
• 1 (10 ounce) package fresh spinach, coarsely chopped
• 10 leaves fresh basil, chopped
• ½ teaspoon red pepper flakes, or to taste
• 1 pinch salt and ground black pepper to taste
• ½ cup grated Parmesan cheese, or as needed