While the meatballs are baking, melt the butter in a large skillet over medium heat. Add the garlic and sauté for one minute.
Turn the heat to medium, add the flour, whisking continuously to create a roux. Cook for a couple of minutes until the roux is golden.
Gradually whisk in the beef broth, ensuring no lumps form. Bring the mixture to a simmer. Add the heavy cream.
Add the Italian seasoning, Dijon mustard, onion powder, and red pepper flakes. Stir to combine.
Add the sliced mushrooms.
Let the sauce simmer for about 5 minutes until it thickens and the mushrooms are tender.
Add the cooked meatballs to the sauce, tossing to coat them evenly. Simmer for 5 minutes.