Fry the bacon in a hot pan until golden brown, take it out and let it drain on kitchen paper. Cut cheese into thick sticks. Cut walnuts in half. Season vinegar with salt, pepper and sugar, add oil.
Peel and finely chop the onion. Fold into the vinaigrette. Clean the endive, pluck into pieces and wash. Break the bacon into pieces. Mix salad, cheese, walnuts and bacon in a bowl and arrange on 4 small plates.
Pour some vinaigrette over it. Add the rest. It goes well with toasted rye bread.