Peel and finely chop the onion. Cut the bacon into cubes and fry it crisply in a hot pan without fat. Add onion and steam briefly. Sprinkle with sugar. Add vinegar while stirring constantly, add olive oil and mix.
Season to taste with salt and pepper and leave to cool. Cook the beans in boiling salted water for about 5 minutes. Then rinse with cold water and drain. Drain the beans on a sieve.
Wash the cucumber and cut into cubes. Wash and halve the tomatoes. Mix beans, cucumber and tomatoes with the cold vinaigrette. Remove the stalk from the radicchio, remove the leaves carefully and wash.
Arrange the salad in the radicchio leaves.