This is variation on the Venetian dish that is made with the long radicchio di Treviso. The rounder radicchio commonly found in the US is more bitter so I’ve combined it the milder Belgian endive.
Put the butter in a 12-inch skillet over medium heat. When the butter has melted, add the onions and Sauté until they turn a rich golden color, about 5 minutes.
Add the bacon and continue cooking until it becomes lightly browned but not crisp. Add the radicchio and end