Preheat oven to 375°F. Coat a rimmed baking sheet with nonstick cooking spray.
In a large mixing bowl, combine ground beef, ground pork, beaten egg, tomato sauce, onion, garlic powder, oregano, basil, salt, pepper, and breadcrumbs. Mix until just combined, avoiding overworking the meat.
Scoop and shape into 1½-inch meatballs (about the size of golf balls). You should get around 20 meatballs. Arrange them evenly on the baking sheet.
Brush the tops with olive oil and bake for 20–25 minutes, or until the internal temperature reaches 165°F.
Remove from oven and top each meatball with a spoonful of marinara and a piece of cheese.
Turn oven to broil and return meatballs to the oven for 2–3 minutes, just until the cheese melts and bubbles.
Serve immediately with toothpicks and extra marinara on the side.