Put butter in a large skillet, cook chicken and onion until cooked.
Add and fry the mixed vegetables, rice and rest of the filling ingredients. Set aside.
Beat eggs and add milk, salt and pepper.
Melt butter in a large skillet. Cook 1/4 egg mixture like a crepe.
Carefully spoon 1/4 of the rice filling over one half of the omelet. Flip the omelet over to cover the rice filling! Repeat with rest of the ingredients.
Drizzle tomato catsup on the omurice if you like.