Heat olive oil in a medium skillet over medium to high heat and add chopped onion. Cook until soft and slightly browned, stirring frequently, about 4-5 minutes. Remove from heat and set aside to cool. In that same pan cook the red pepper, red onion and mushrooms until soft and slightly brown, 5-6 minutes reserve until ready to assemble the subs.
In a large bowl, combine the ground turkey, bread crumbs, parsley, egg, black pepper and oregano. Add the cooked, cooled chopped onions and mix all the ingredients well. With wet hands, make 24 meatballs, about 1/4 cup each, place on a baking sheet and put in the fridge until sauce is ready.