Begin by bringing a large pot of salted water to a boil. Add the tortellini and cook according to the package instructions until al dente. This usually takes about 3-5 minutes for refrigerated tortellini or 7-10 minutes for frozen. Once cooked, drain the tortellini in a colander and set aside.
In a large sauté pan, melt the butter over medium heat. Add the diced onion and cook until translucent, about 3-4 minutes. Stir in the minced garlic and cook for another 1-2 minutes until fragrant.
Next, add the thawed and drained spinach to the pan. Mix well and cook for about 2 minutes, allowing the flavors to meld. Then, stir in the diced tomatoes, dried basil, salt, and black pepper. Let everything simmer for an additional 4-5 minutes.
Pour in the heavy cream, stirring to combine all ingredients. Allow the mixture to simmer gently for about 3-4 minutes, until it thickens slightly.
Add the cooked tortellini to the creamy spinach mixture, tossing gently to coat all of the pasta with the sauce. Lastly, sprinkle freshly grated Parmesan cheese over the top, stirring until the cheese melts into the sauce. Serve immediately, garnished with extra Parmesan if desired.
