The apple in this Korean bulgogi-inspired sauce provides body, sweetness and a bit of acid to tenderize the steak tips. We make enough of it to marinate the steak and serve as a dipping sauce.
Position racks in upper and lower thirds of oven. Place a large rimmed baking sheet on the lower rack. Preheat oven to 425°F. Line another baking sheet with foil.
Toss potatoes with 2 tablespoons oil and 1/8 teaspoon each salt and pepper in a medium bowl. Carefully transfer to the hot baking sheet. Roast on the lower rack, flipping once, until the potatoes are browned and tender, about 35 minutes.
Meanwhile, combine apple, onion, scallion whites, garlic, brown sugar, gochujang, soy sauce and vinegar in a blender. Puree until smooth. Reserve 1/2 cup for serving and pour the rest into the bowl; add steak tips and toss to coat. Marinate at room temperature for 10 minutes.
Combine broccolini, the remaining 1 tablespoon oil and 1/8 teaspoon each salt and pepper in a medium bowl; toss to coat.
Arrange the broccolini on one side of the foil-lined baking sheet and the steak on the other side. Roast on the upper rack until the broccolini is tender and an instant-read thermometer inserted in the thickest part of the steak registers 125°F for medium-rare, 10 to 14 minutes. Turn the broiler to high and broil until the broccolini and steak start to brown, 1 to 2 minutes.
Serve the steak with the broccolini, potatoes and reserved sauce. Garnish with scallion greens.