In a large skillet, heat a tablespoon of oil over medium heat. Add diced chicken and finely chopped onions, cooking for about 5-7 minutes until the chicken is golden and fully cooked. Stir in the diced vegetables and sauté for another 2-3 minutes until they soften. Finally, add in the cooked Japanese short-grain rice and ketchup, mixing thoroughly until heated through and well combined.

In a mixing bowl, whisk together 3-4 fresh eggs until fully blended and slightly frothy. Heat a non-stick frying pan over low heat and add a drizzle of oil. Pour the egg mixture into the pan, swirling it around for a thin, even layer. Cook for approximately 2-3 minutes until the edges start to set but the center remains soft and creamy.

Once the omelette is set on the outside but still soft inside, spoon a generous portion of the chicken rice onto one half of the omelette. Gently fold the empty half over the filled side.

Allow the filled omelette to sit for about 1-2 minutes on low heat to help it stabilize. Carefully slide the beautifully folded omurice onto a plate, and drizzle extra ketchup or a sauce of your choice over the top.