The sauce is also delicious served with pork chops or roasted chicken.
Preheat the oven to 450°. Thinly slice the potatoes crosswise, keeping the bottom 1/4 inch intact. Transfer to a baking sheet. Brush the potatoes generously with half the melted butter; season all over with salt and pepper. Bake until crisp at the edges, about 50 minutes. Brush the potatoes with the remaining melted butter. Bake until tender, about 10 minutes more.
In a medium skillet, melt the remaining 2 tbsp. butter over medium heat. Add the garlic. Cook, stirring often, until fragrant, 3 to 5 minutes. Using a slotted spoon, transfer the garlic to a plate. Add the onion to the skillet. Cook, stirring often, until golden, 10 to 15 minutes.
In a medium saucepan, boil the stock over high heat until reduced to 1/2 cup, about 15 minutes. Stir in the oyster sauce, cornstarch, soy sauce, and coarsely ground black pepper. Bring to a boil. Cook until thickened, about 2 minutes. Stir in the vinegar, garlic, and onion; season the sauce.
Heat a grill or grill pan over high. Season the steaks. Grill the steaks to desired doneness, 3 to 4 minutes per side for medium-rare.
Arrange the steaks and potatoes on plates. Top the steaks with the sauce and scallions.