Make horseradish condiment in advance by combining butter and chopped horseradish in a small bowl. Add 2 tbsp of lemon juice and beat well. Place on waxed paper and form into a 1-1/2 to 2-inch diameter roll. Wrap and refrigerate until firm. When ready to serve, cut into slices and place on hot meat.
Combine the remaining lemon juice, olive oil, garlic, salt and pepper in a small mixing bowl, mix well and pour over steak inside of a large plastic bag. Close bag securely, coating meat fully. Marinate overnight if possible.
Place grids at high level and preheat grill to medium high. Pour remaining marinade into a saucepan and heat to boil; reduce heat. Grill steak on one side for 10 to 15 minutes and then turn. Brush with remaining marinade; turn and grill for similar times until degree of doneness is achieved. Place finished London broil on a carving board and carve into strips. Pour the boiled marinade over sliced strips and add one or more slices of horseradish condiment on top of steak. Serve with salad and vegetables as desired.
