Bring a large pot of salted water to a boil and cook the tortellini according to package directions; reserve 1/2 cup of the pasta cooking liquid, then drain the tortellini.
Meanwhile, melt the butter in a large skillet over medium heat.
Add the diced onion and cook until softened, about 3 minutes.
Add the minced garlic and cook until fragrant, about 1 minute.
Stir in the drained tomatoes and the well-drained chopped 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 cooked tortellini and toss to coat; add reserved pasta water a little at a time if the sauce is too thick.
