Omurice – Japanese Rice Omelette

Name: Omurice – Japanese Rice Omelette
Ingredients: 0.5 onion (thinly sliced); 1 boneless chicken thigh (cut into bite-size pieces); 1 tablespoon neutral oil; 2 stalks spring onions (thinly sliced); 1 pinch salt; 1 pinch black pepper; 2 servings cooked sushi rice (day-old, about 300 g); 1 tablespoon ketchup (plus extra for drizzling); 1 teaspoon light soy sauce; 1 teaspoon oyster sauce; 2 eggs; 2 tablespoons milk (divided); 2 tablespoons neutral oil (divided); 6 tablespoons cheddar (grated)