In a large skillet, heat 1 tablespoon of oil over medium heat. Sauté 1 diced onion and 1 cup of mixed vegetables for about 3–4 minutes until softened. Add 1 cup of diced chicken and cook until lightly browned, about 5–7 minutes. Stir in 2 cups of cooked Japanese short-grain rice and ¼ cup of ketchup. Combine well, allowing the mixture to heat through, about 2–3 minutes.

In a bowl, whisk together 4 eggs and season lightly with salt. Heat a non-stick skillet over medium-low heat and add a small amount of oil, swirling to coat. Pour in the egg mixture, tilting the pan to spread it evenly. Cook for 2–3 minutes until the edges set but the top remains slightly runny.

Once the omelette is set around the edges, spoon the chicken rice mixture into the center. With a spatula, gently fold the sides of the omelette over the rice. Allow it to cook for another minute to ensure the rice is warmed through.

Carefully slide the wrapped Omurice onto a plate, seam side down. Optionally, drizzle with extra ketchup or a red wine sauce and sprinkle with freshly grated parmesan cheese if desired.