2 tablespoons extra-virgin olive oil
1 medium fennel bulb, trimmed, halved lengthwise and cut crosswise into 1/4-inch-thick slices (about 3 cups)
1 medium red onion, 1/4-inch-thick slices (about 2 cups)
1 can white beans, drained and rinsed
1 cup vegetable to chicken stock
2 teaspoons fresh thyme
Coarse salt and freshly ground pepper
2 tablespoons red-wine vinegar
1 tablespoon unsalted butter, softened
Grated parmesan cheese (optional)