Chop the onion finely.Cut the chicken into small cubes.Heat the oil in a non-stick pan and sauté the onion until softened.Add the chicken and cook until no longer pink.Add the mixed vegetables and season with salt and pepper.Add the rice and break into small pieces.Add ketchup and soy sauce and combine everything evenly with a spatula. Transfer the fried rice to a plate.Whisk the egg and milk together in a small bowl.Heat the oil in the pan over medium high heat.When the pan is hot, pour the egg mixture into the pan and tilt to cover the bottom of pan. Lower the heat when the bottom of the egg has set (but still soft on top).Put the fried rice on top of the omelette.Use the spatula to fold both sides of omelette toward the middle to cover the fried rice. Slowly move the omurice to the edge of the pan.Move the omurice to the plate.Drizzle the ketchup on top for decoration and serve with lettuce.