Boil a pot of water and cook the tortellini according to the package directions. Drain the tortellini and set aside.
 In a large sauté pan over medium heat, melt the butter. Add the onion and minced garlic and cook for 2 minutes, stirring frequently, until translucent.
 Add the basil, tomato sauce, diced tomatoes, heavy cream and simmer for a few minutes, until the sauce starts to bubble.
Add the spinach, and fold into the sauce and cover until all the spinach has cooked down stir occasionally.
Fold in the Tortellini until everything is coated.