Sauté the base: Heat olive oil in a large pot over medium heat. Add onion, carrots, and celery with a pinch of salt. Cook 5–7 minutes, stirring, until softened and lightly golden.
Add aromatics: Stir in garlic, Italian seasoning, and red pepper flakes.
Cook 30–60 seconds until fragrant, taking care not to burn the garlic.
Build the broth: Add crushed or diced tomatoes and broth. Drop in a bay leaf and a Parmesan rind if using. Bring to a gentle boil, then lower to a simmer for 10 minutes to let flavors meld.
Cook the tortellini: Increase heat to a gentle boil.
Add tortellini and cook according to package directions, usually 3–5 minutes for fresh, a bit longer for frozen. Taste the broth and season with salt and pepper.
Stir in spinach: Turn off the heat and fold in spinach. It will wilt in about 1 minute.
If you’re using sturdier greens like kale, simmer them earlier for 5–10 minutes before adding tortellini.
Finish and serve: Remove the bay leaf and Parmesan rind. Squeeze in a little lemon juice if you like brightness. Ladle into bowls and top with chopped parsley or basil and a shower of grated Parmesan.