Melt butter in a large frying pan over medium high heat and add onion and tomatoes. Cook until onion is translucent and tomatoes are soft. Add salt and pepper to taste.
Now add your cheese, a few pieces at a time until entire block has been added. Stir gently so the cheese does not burn and start to stick to the pan. 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!