Let the steak rest at room temperature for about 20 minutes, then pat dry with paper towels. Cut into bite-sized cubes and season with salt, pepper, and red pepper flakes for a little heat.
Peel and chop the potatoes into chunks, cover with cold salted water, and bring to a boil. Cook until fork-tender, about 15–20 minutes.
Heat olive oil in a hot skillet and sear the beef in a single layer for 2–3 minutes per side until golden brown. Work in batches if needed.
Lower the heat, add butter, and let it melt until foamy. Stir in minced garlic and Worcestershire sauce, cooking just until fragrant. Toss the beef so each piece is coated in the buttery sauce.
Drain the potatoes and mash them with butter, milk or cream, salt, pepper, and garlic powder. Add more milk if needed for extra creaminess.
Spoon the mashed potatoes onto plates, pile the garlic butter beef bites on top, and drizzle with the remaining sauce. Sprinkle fresh parsley for a bright finish.