While the potatoes are cooking, make the filling: Heat the EVOO in a large, deep skillet over medium-high heat until it shimmers. Add the meat and brown it well, about 5 minutes. Add the vegetables and bay leaf to meat, season with salt and pepper and cook until the vegetables are softened, 8-10 minutes. Sprinkle the flour around the pan, stir and cook for about 1 minute. Add the stock and bring up to a simmer to thicken the gravy, 3-4 minutes. Discard the bay leaf and season the gravy with Worcestershire sauce, salt and pepper.
Turn broiler on and place rack in the middle of the oven.
Transfer the filling to a medium-size casserole or baking dish and top it with heaping spoonfuls of the horseradish mashed potatoes so they completely cover it. Brown the potatoes under the broiler, about 2 minutes, garnish with chives and serve.