Preheat the oven to 375 degrees F (190 degrees C). Line a rimmed baking sheet with foil. Spray foil lightly with cooking spray.
Combine onion, bread crumbs, half-and-half, Parmesan cheese, eggs, egg yolk, garlic, basil, parsley, and oregano in a large bowl; stir until well combined. Add ground round and sausage and sprinkle evenly with salt and pepper. Mix well until just combined.
Dampen hands with water and form mixture into 18 golf ball-sized meatballs. Arrange meatballs on the prepared baking sheet.
Bake in the preheated oven until browned, 12 to 15 minutes.
Transfer meatballs to a large pot and add marinara sauce. Simmer over low heat for at least 2 hours before serving.