Step 1: Heat 2 tablespoons of olive oil in a large pot or Dutch oven over medium heat. Add the finely diced yellow onion and sauté until softened and translucent, about 5-7 minutes. Stir frequently.
Step 2: Add the minced garlic to the pot and cook for another minute until fragrant. Be careful not to burn the garlic.
Step 3: Pour in the chicken or vegetable broth and crushed tomatoes. Stir in dried basil, dried oregano, and red pepper flakes (if using). Season generously with salt and freshly ground black pepper. Bring the mixture to a gentle simmer, then reduce heat to low, cover, and let it cook for at least 15 minutes to allow the flavors to meld.
Step 4: Increase the heat back to medium. Add the tortellini to the simmering soup. Cook according to package directions, typically 3-5 minutes for fresh tortellini, or a few minutes longer for frozen, until the tortellini are al dente and float to the surface. Avoid overcooking.
Step 5: Stir in the heavy cream (or half-and-half) and the fresh spinach. Cook for another 1-2 minutes, just until the spinach wilts. Do not boil the soup after adding the cream.
Step 6: Remove the pot from the heat. Stir in the grated Parmesan cheese until melted and fully incorporated. Taste and adjust seasonings one last time. Serve hot, garnished with extra Parmesan cheese and fresh chopped parsley or basil.