Bring a large pot of salted water to a boil and cook the tortellini according to package directions; before draining, reserve 1/2 cup of the cooking liquid, 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.
Add the minced garlic and cook until fragrant, about 1 minute.
Stir in the drained diced tomatoes and thawed, squeezed-dry spinach and cook for about 3 minutes to heat through.
Add the basil, salt, pepper, and heavy cream; bring to a gentle simmer and cook for about 5 minutes to thicken slightly.
Remove the skillet from the heat, stir in the Parmesan until melted, then add the drained tortellini and toss to coat; if the sauce is too thick, stir in reserved pasta cooking liquid a little at a time to reach desired consistency.