Dice chicken into bite-sized pieces and mince the onion. Microwave the onion at 600W for 30 seconds.

Heat butter in a frying pan, add onion and chicken, and sauté until the chicken changes color.

Add ketchup and salt and pepper, mix well, then add the rice and stir to combine.

In a microwave-safe bowl, combine flour and butter, and microwave at 600W for 20 seconds.

Gradually add milk to the flour mixture, stirring continuously. Microwave at 600W for 3 minutes.

Place the chicken rice in a heatproof dish, top with white sauce and cheese.