1 tablespoon butter
2 teaspoons olive oil (divided)
2 medium red onions (halved and sliced thinly, about 5-1/2 cups)
1 large heads of Chioggia or Treviso radicchio (cored and sliced thinly, about 8 cups)
3 tablespoons balsamic vinegar
4 fully cooked chicken-apple sausages (about 3 ounces each)
  Salt and freshly ground pepper to taste