Mix breadcrumbs and milk in a bowl and let soak.

Remove the stem from the tomato and cut it in half horizontally.

Mince the onion and microwave in a heatproof container for 1 minute at 600W. Let cool.

Add ground meat, salt, and pepper to the breadcrumb mixture. Mix until sticky and divide into two oval patties.

Heat salad oil in a pan over medium heat. Add patties and press the center slightly to create a dent. Cook until browned, flip, and add tomatoes cut-side down. Cover and cook on medium-low heat for 7 minutes.

Place cheese slices on the patties and remove from the pan.

Wipe the pan clean with a paper towel. Add tonkatsu sauce and ketchup, stirring over medium heat until thickened.