Heat the butter in a large pot over medium heat until melted and hot.
Add the diced onion, minced garlic, kosher salt, and black pepper, cooking and stirring frequently until the onion softens and turns translucent, about 6 minutes.
Pour in the balsamic vinegar and stir well.
Incorporate the crushed tomatoes, broth, heavy cream, and bay leaves, then raise the temperature to bring the mixture to a rolling boil.
Turn the heat down to low and let the soup gently simmer for 5 minutes to blend the flavors.
Introduce the tortellini to the pot, continuing to cook at a simmer until the pasta is tender and cooked through, approximately 5 to 6 minutes (fresh tortellini may only need about 3 minutes).
Remove the soup from heat; fish out and discard the bay leaves.
Stir in the sliced fresh basil leaves.
Serve the soup hot, garnished with grated Parmesan cheese and additional basil if desired.
