Clean the radicchio, remove the outer leaves, cut in half lengthways and separate all leaves from the white stalk. Remove the white midrib from all leaves. Rinse the leaves briefly and place in water. Remove the bitter substances for 15 minutes, then strain and drain well.
In the meantime, cut the bacon into thin strips. Cap the onion at both ends, peel and quarter lengthways. Halve the larger, outer quarters lengthways. Cap the garlic at both ends, peel and cut lengthways into strips.
Heat the olive oil in a larger pan. Add the lean bacon and fry for 2 minutes. Reduce the heat and add the onion skins, stir-fry for approx. 2 minutes until translucent. Mix in the garlic cloves. The heat supply to the lowest level or switch off. As soon as the temperature has dropped below 80 degrees, add the radicchio leaves. Fry for about 3 minutes, turning twice. Carefully season with a little salt and a little more freshly ground pepper (see note).
Drizzle the sauce over the lettuce leaves and serve warm in the pan as a side dish or starter.