In a bowl, whisk together the soy sauce, mustard, garlic, oregano, and House Seasoning. Slowly whisk in the vegetable oil until fully incorporated.
Add the steak to the marinade. Cover and let sit for at least 1 hour and as long as overnight, refrigerated.
Heat the grill to medium-high. Grill the steak, turning once, for 4 to 5 minutes per side for medium rare. Transfer the steak to a serving platter to rest. Spread the baguette slices with butter, and grill for about 1 to 2 minutes per side. Set aside in a serving bowl.
Slice the steak and serve with prepared horseradish sauce and the baguette slices.
Mix all the ingredients together and store in an airtight container for up to 6 months.