Boil the tortellini: Bring a large pot of salted water to a gentle boil. Add tortellini and cook until they float and are just tender, usually 2–4 minutes. Reserve 1/2 cup pasta water, then drain.

Sauté aromatics: In a large skillet, heat olive oil or butter over medium. Add onion and cook until soft, about 4–5 minutes. Stir in garlic for 30 seconds, just until fragrant.

Build the sauce: Pour in crushed tomatoes or marinara. Add Italian seasoning, a pinch of red pepper flakes if using, and a light sprinkle of salt and pepper. Simmer for 5–7 minutes to meld flavors.

Make it creamy (optional): Stir in the cream or half-and-half. Simmer 1–2 minutes more. If the sauce feels thick, loosen with a splash of reserved pasta water.

Add greens and cheese: Stir in spinach until just wilted, about 1 minute. Fold in Parmesan until the sauce turns silky.

Toss with tortellini: Add drained tortellini to the skillet and gently toss to coat. If needed, add more pasta water for a glossy finish. Taste and adjust salt and pepper.

Finish and serve: Top with extra Parmesan and chopped basil or parsley. Serve hot with crusty bread or a simple salad.