Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin tin with the olive oil.
Add the ground beef, Italian sausage, egg, breadcrumbs, Parmesan, onion, Italian seasoning, garlic powder, salt, pepper, and ¼ cup of the marinara to a large bowl. Mix gently until the ingredients are evenly combined. Avoid overmixing so the muffins stay tender.
Divide the mixture among the 8 greased muffin cups, pressing it in lightly. Spoon the remaining marinara over the tops.
Bake for 25 to 30 minutes, until the meatloaf muffins are cooked through and no longer pink in the center. Let them rest in the tin for 5 minutes before removing.
While the muffins bake, slice the polenta into 8 rounds. Warm a nonstick skillet over medium heat and cook the rounds for 3 to 4 minutes per side, until heated through and lightly golden.
Serve two meatloaf muffins with two polenta rounds per person. Spoon any marinara remaining in the muffin cups over the muffins.