Braised Fennel with Cannellini and Bacon

Name: Braised Fennel with Cannellini and Bacon
Ingredients: 8 slice bacon, sliced cross wise into ½ pieces; 1 medium fennel bulb, trimmed, halved lengthwise and cut crosswise into ¼‑inch-thick slices (about 3 cups); 1 medium red onion, trimmed, halved lengthwise and cut crosswise into ¼‑inch-thick slices (about 2 cups); 1 can (15 ½ ounces) white beans; 1 chicken broth; 2 teaspoon coarsely chopped fresh oregano, plus whole leaves for garnish; ¼ teaspoon red pepper flakes; coarse salt and freshly ground pepper; 2 tablespoon red wine vinegar; 1 tablespoon unsalted butter, softened