In a large pot or Dutch oven, heat the olive oil over medium heat. Add the diced onion and cook until softened, about 3–4 minutes. Stir in the minced garlic and cook for another 1–2 minutes until fragrant.
Pour in the chicken or vegetable broth and bring to a simmer. Stir in the dried basil and crushed red pepper flakes (if using) for added flavor.
Add the cheese tortellini to the simmering broth and cook according to the package instructions, usually about 5–7 minutes. Fresh tortellini will cook faster than frozen.
Reduce the heat to low and stir in the heavy cream. Add the chopped spinach, allowing it to wilt into the soup. Simmer gently for 2–3 minutes to combine the flavors.
Stir in the grated Parmesan cheese and season with salt and pepper to taste. Adjust the seasoning as needed.
Ladle the soup into bowls and garnish with extra Parmesan cheese and freshly cracked black pepper. Serve with crusty bread for a complete meal.
