Pat the meat dry. Sprinkle it with kosher salt and freshly ground pepper, then dredge in flour.
Heat 2 tablespoons oil in a large Dutch oven over medium-high heat. Once hot and ripping, add half of the meat and brown, then remove to a plate. Repeat with the second batch of meat. Once all the meat is evenly browned, reduce the heat a bit and add another drizzle of oil, if necessary. Add the onions, garlic and bay leaf and cook to soften, 7-8 minutes. Add the tomato paste and stir for a minute. Stir in the beer and reduce by half, a minute or two more. Add the Worcestershire sauce, beef stock, horseradish and beef. Add just enough water to surround the meat and partially cover. Simmer over medium-low heat for 2 hours, until very tender. Cool and store in the fridge for a make-ahead meal.
To serve, quarter the potatoes. Cover with water in a medium size pot and bring to boil. Salt the water and cook to just-tender, 10-12 minutes, tops. Drain and return to the hot pot. Add the butter and parsley and stir to melt the butter.
Place a small pile of buttered potatoes in each shallow bowl, top with stew and garnish with watercress leaves. Serve with pumpernickel or sourdough bread.
Cook’s Note: When storing bread in your kitchen for a few days, wrap tightly in plastic to keep tender, and crisp in a hot oven. Do not refrigerate.