4 tablespoons unsalted butter
3 pounds beef flatiron or blade steaks, cut into 1/3-inch-thick slices, about 3 inches wide
Salt and freshly ground pepper
3 cups thickly sliced onions
1/2 cup all-purpose flour
Three 12-ounce bottles Belgian ale
1/2 teaspoon dried thyme
2 bay leaves
Chopped parsley, for garnish
Boiled carrots and potatoes, for serving
In an enameled cast-iron casserole, melt 2 tablespoons of the butter. Season the beef with salt and pepper and add one-third of it to the casserole. Cook over moderate heat until lightly browned, 3 minutes per side. Transfer to a bowl. Repeat with 2 more batches of meat, using the remaining 2 tablespoons of butter.
Add the onions to the casserole, cover and cook over low heat, stirring, until browned, 8 minutes. Stir in the flour until the onions are well-coated, then slowly add the beer. Return the meat to the casserole along with any accumulated juices. Add the thyme and bay leaves, cover and simmer over low heat, stirring, until the beef is tender, 2 hours.
Uncover and transfer the meat to a bowl. Simmer the sauce over moderate heat until thickened slightly. Discard the bay leaves. Return the meat to the casserole and season with salt and pepper. Sprinkle with parsley and serve with boiled carrots and potatoes.
MAKE AHEAD The carbonnade can be refrigerated for up to 3 days. Reheat gently.