Cut the slice of bread in half.
Slice the onion thinly. Take the stem end and seeds off the green pepper, cut in half and slice thinly lengthwise. Slice the bacon into thin strips.
Put the Step 2 mixture in a heatproof bowl, cover with plastic wrap and microwave for about a minute (at 600 W). Mix in the ketchup.
Put 1/2 of each topping mixture on each half of the bread (this way, it's easier to separate the slices when eating them!)
Line the oven rack with a piece of foil that has been crinkled up. Put the bread on top of the foil with the two halves right next to each other. Place the cheese on top.
If you use melting-type sliced cheese, cut it in half before putting it on the bread. That way it will be easier to eat.
When the cheese has melted nicely, it's done.
If you wrap bacon in single-use portions and freeze it, it's convenient to use.