Preheat oven to 275° F.
Heat a Dutch oven over medium heat until hot. Add oil and butter and heat.
Sear brisket on all sides until deep golden brown, about 3 minutes per side. Pour beer over the beef. Add enough water to come halfway up the meat.
Wrap the peppercorns and bay leaves in a square of cheesecloth and place in the Dutch oven with the meat. Cover the pot tightly, and braise for 2½ hours, or until meat is tender.
Remove the peppercorns and bay leaves.
Place the potatoes around the meat, followed by the garlic, carrots and cabbage. Add enough water to barely cover. Bring to a simmer on the stove top, then cover the Dutch oven and braise until vegetables are tender, about 30 minutes.
To serve, drain the vegetables with a spider or a slotted spoon, and arrange them on a serving platter. Drizzle melted butter over the vegetables.
Thinly slice the beef across the grain and place on top of the vegetables. Garnish with minced parsley and lightly sprinkle with just a pinch or two of Fleur de Sel.