Clean, wash, spin-dry the lettuce and pluck it into larger pieces. Grate the mushrooms and cut into slices.
Leave the diced bacon in a hot pan and fry until crispy, remove from the fat and drain on kitchen paper. Fry the mushrooms in the bacon fat until crispy, also remove and set aside.
Peel and finely dice the onion. Stir a vinaigrette from oil, vinegar, mustard, onions, salt and pepper. Fold in the mushrooms and finally sprinkle with the bacon.