Preheat broiler. Take the steak out of the refrigerator and let it come up to room temperature.
In the meantime, heat the EVOO, about 2 turns of the pan, and 2 tablespoons of butter in medium saucepot over medium-high heat. Once the butter is melted, add the mushrooms and cook about 5 minutes, until brown.
Add the onion, garlic, carrots, parsnips and herbs. Cook until veggies are tender, about 4-5 minutes. Add the potatoes and 3 cups of stock plus one cup of water. Bring up to a bubble then reduce heat and simmer 20-30 minutes.
As the liquid simmers, place the London broil on a broiler pan. Rub it with about 2 tablespoons of EVOO and season it generously with salt and freshly ground black pepper.
Place the meat under the broiler about 6 inches from the flame for 10-15 minutes, flipping it over halfway through to ensure even cooking. Remove the meat from the broiler and let it rest for about 5 minutes.
While the meat is resting, add the remaining 2 tablespoons of butter to a small skillet over medium-high heat. Once melted, add flour and cook about 1 minute. Whisk in the remaining stock, about a cup, the add the Worcestershire sauce and Dijon mustard. Once thickened, add the mixture into your saucepot with the veggies.
Ladle the veggie stew into shallow bowls. Cut the steak into thin slices and place it on top. Serve with a dollop of prepared horseradish on top and a nice hunk of crusty bread with butter on the side.