Cut the onions and peppers into circles, stew them in vegetable oil until soft.
Add a little water. Allow to soak in a marinade of the lemon juice, ketchup, salt and pepper overnight (if you don’t have time, 1 hour will do).
Shortly before serving, put the onion and peppers from the marinade on the toasted white bread slices, cover with a slice of hard feta cheese and put in a preheated oven.
As soon as the cheese melts, take the sandwiches out and serve.