Heat 2 teaspoons vegetable oil in a large frypan. Cook and stir onion over medium heat until onions begin to brown.
Combine tuna, sour cream, cream cheese, mozzarella cheese, olives, dill, garlic powder and cooked onion in a large bowl.
Beat eggs with milk in a large bowl. Heat 2 teaspoons of oil in the frypan used to cook the onions. Pour the egg mixture into the hot pan. As eggs set, lift the edges to allow the liquid to run underneath for even cooking. When eggs are almost fully set, spoon tuna mixture onto one half of the eggs; fold the untopped half over the filling. Cover pan, and remove from heat. Allow omelette to rest in covered pan until the cheese is melted.