Trim and cut the steak into 1-inch cubes. Pat dry with paper towels. Season with salt, pepper, and optional red pepper flakes.
Heat olive oil in a hot skillet. Add beef in a single layer, sear 2–3 minutes per side until golden. Work in batches if needed.
Reduce heat. Melt butter, stir in garlic and Worcestershire sauce. Scrape the pan and cook for 1 minute until fragrant.
Return beef to the pan, toss in the garlic butter, and cook for 1–2 minutes. Sprinkle with parsley.
Place chopped potatoes in salted cold water. Bring to a boil, then simmer 15–20 minutes until fork-tender.
Drain well, return to pot, and mash using your preferred method.
Mix in butter, warm milk or cream, salt, pepper, and garlic powder. Stir until smooth.