Heat olive oil in a large skillet over medium heat. Add garlic and onion. Sauté until soft, about 3-4 minutes.
If you're using chili flakes, toss them in now and give everything a stir.
Pour in the cream and bring to a gentle simmer. Stir in parmesan, salt, and pepper. Let it thicken slightly.
Add the tortellini straight from the package—no need to pre-cook. Let them simmer in the sauce for 5-6 minutes until tender.
Stir in the spinach if using, and cook just until wilted. Taste and adjust seasoning.
Serve hot, topped with fresh basil and maybe more parmesan (go on, you deserve it).