Melt the butter in a small frying pan and brown the bread in it on both sides. Leave to cool.
Wash the lettuce, dry well and place on the bread. Top with the Tilsit or Cheddar.
Drain the sardines, lay them diagonally across the cheese and arrange the sliced tomato on top.
Peel and finely dice the onion. Sprinkle on the tomato, followed by the salt, pepper and grated Emmental or Edam cheese.
Danish Sandwiches can be topped with canned anchovy fillets cut into strips instead of sardines. Soak the anchovies first in a little milk, as they tend to be very salty.