Bring a large pot of salted water to a boil and cook the tortellini according to package directions. Reserve 1/2 cup of the cooking water, then drain the pasta.
Meanwhile, melt the butter in a large skillet over medium heat.
Add the diced onion to the skillet and cook until softened, about 3 minutes.
Stir in the minced garlic and cook for 1 minute until fragrant.
Add the drained diced tomatoes and the thawed, well-squeezed spinach; cook for about 3 minutes to heat through.
Stir in the basil, salt, pepper, and heavy cream. Bring to a gentle simmer and cook for about 5 minutes to slightly thicken the sauce.
Remove the skillet from the heat and stir in the grated Parmesan until melted and smooth.
Add the cooked tortellini to the skillet and toss to coat. If the sauce is too thick, add some of the reserved pasta cooking water a tablespoon at a time until desired consistency is reached.