Dice onion and tomatoes, put to one side. Slice cheese into chunks to make easy for melting, put to one side.
Melt butter in a large frypan over medium/high heat and add onions and tomatoes. Cook until onions are transparent and tomatoes are soft. Add salt and pepper to taste.
Now add your cheese, a few pieces at a time until the lot has been added. Stir gently so the cheese does not burn and start to stick to the frypan. Cook for 10 minutes or until cheese is all melted. While you are waiting for this, toast your bread. Once toast and cheese mix is all done, all you have to do is spoon the cheese mix onto the toast slices and serve immediately! Enjoy!