Cut four slices out of the sausage. Fry the sliced sausages in a grill or a pan so that they get crispy on the surface. Toast the buns. Mix mustard and mayonnaise and spread on the bottom of the buns. Put the sausage slices on top. Slice the onions ans tomatoes thinly and put on the sausage. Grind some black pepper and a pinch of salt on the fillings. Spread some ketchup on the other half of the bun and place on top. Enjoy!