Fry the bacon until crispy, take it out. Wash the meat, dab dry. Fry in the frying fat for approx. 6 minutes on each side. Season with salt and pepper.
Clean, wash and drain the salad. Peel onion and cut into rings. Wash and slice the tomatoes. Mix mayonnaise and mustard, season with salt, pepper and lemon juice.
Toast in the toaster. Cut the meat open. Spread 4 toasts with mustard cream. Pile lettuce leaves, meat, onion cubes, tomatoes and cheese on top. Spread the rest of the toast with butter and place on top.