Brown the sausage: Heat a large pot or Dutch oven over medium-high heat. Add olive oil if the sausage is very lean.
Crumble in the turkey sausage and cook, breaking it up with a spoon, until browned and cooked through, about 5–7 minutes. Spoon off excess fat if needed.
Sweat the aromatics: Add onion, carrots, and celery. Season with a pinch of salt.
Cook, stirring occasionally, until softened, 5–6 minutes. Add garlic, Italian seasoning, and red pepper flakes. Cook 30–60 seconds until fragrant.
Build the broth: Stir in diced tomatoes, chicken broth, and the bay leaf.
Scrape up any browned bits from the bottom of the pot. Bring to a gentle boil, then reduce to a steady simmer for 10–12 minutes to meld flavors.
Cook the tortellini: Add the tortellini to the simmering soup. Cook according to package directions, usually 3–5 minutes, until just tender.
Avoid overcooking—they’ll continue to soften in the hot broth.
Add greens and cheese: Stir in spinach (or kale). Simmer 1–2 minutes until wilted. Turn off the heat and stir in Parmesan.
Taste and season with salt and pepper. If desired, squeeze in a little lemon for brightness.
Serve: Ladle into bowls and top with more Parmesan and herbs. Serve hot.