In a large skillet over medium heat, heat 1/2 cup oil. Arrange potatoes in a single layer with a cut side facing down; season with 1/2 teaspoon salt. Cook, undisturbed, until one side starts to brown, about 10 minutes. Toss potatoes so another cut side is facing down and add smashed garlic. Cook, undisturbed, until both sides are golden brown, 10 to 12 minutes more. Transfer potato mixture to a paper towel-lined plate.

Meanwhile, in a small saucepan over medium heat, melt butter. Once bubbling subsides, add chopped garlic and cook, stirring, until fragrant, about 2 minutes. Add compound beef butter seasoning and cook, stirring, until warmed through, about 1 minute more. Transfer butter mixture to a small bowl. Add mustard, Worcestershire sauce, and red pepper and vigorously whisk until smooth and emulsified.

Season steak with 1 teaspoon salt and 1/2 teaspoon black pepper. In same skillet over medium heat, heat 1 1/2 teaspoons oil. Arrange half of steak in skillet in a single layer and cook, undisturbed, until bottom starts to brown, about 2 minutes. Toss steak and continue cook until desired degree of doneness is reached and other side is starting to brown, 1 to 2 minutes more. Transfer steak to a plate. Repeat with remaining 1 1/2 teaspoons oil and steak. Return potatoes and steak to skillet and remove from heat.

Drizzle steak and potatoes with mustard butter sauce.