Preheat your oven to 400°F (200°C).

In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, egg, garlic, parsley, salt, and pepper. Mix until well blended.

Shape the mixture into 1-inch meatballs and place them on a baking sheet lined with parchment paper.

Bake the meatballs in the preheated oven for 20-25 minutes, or until they are cooked through.

Meanwhile, heat the olive oil and butter in a large skillet over medium heat.

Add the chopped onion and sliced mushrooms to the skillet and sauté until the onions are translucent and the mushrooms are tender.

Stir in the beef broth and bring the mixture to a simmer.

Reduce the heat to low and mix in the heavy cream. Cook for an additional 5 minutes, stirring occasionally.

Thicken the sauce by adding the cornstarch mixture while stirring. Allow it to simmer until it thickens to your desired consistency.

Once the meatballs are cooked, add them to the skillet with the sauce and gently stir to coat them evenly.

Serve the meatballs hot, drizzled with the creamy mushroom sauce.

For a lighter version, you can substitute ground turkey or chicken for the beef. If you have mushroom skeptics at your table, consider blending the mushrooms into the sauce for a smoother texture that hides the mushroom pieces. You can also add a handful of spinach to the sauce for extra nutrition. For those who love a spice, a pinch of crushed red pepper can add a nice kick to the sauce.