Preheat oven to 400 degrees F. Dice onion. On one side of a sheet pan, put the onion. On the other half, place the cremini mushrooms. You can leave the mushrooms whole. I prefer to remove the stems. Drizzle with oil, garlic powder, dried basil, and dried oregano. Toss to coat, keeping mushrooms and onion separate.
Roast until softened and brown, about 20 minutes. Let cool.
In a food processor, place mushrooms and process until finely chopped. Add onion, fresh parsley, basil, garlic, salt, black pepper, and red pepper flakes. Process a few times to combine.
Transfer mushroom mixture to a bowl. Add ground beef and use hands to gently combine. Form meatballs and place on the same sheet pan used to cook mushrooms and onions (less to clean!).
Reduce oven to 350 degrees F. Bake meatballs until cooked through, an internal temperature of 165 degrees F, about 30-40 minutes.
Use meatballs on pasta, for a meatball sub, or however you like!