Bring a pot of salted water to a boil and add the peeled, chopped potatoes; cook until tender, about 15 minutes. Drain and set aside.
While the potatoes cook, pat the steak chunks dry with paper towels to reduce splatter and ensure a good sear.
Heat a skillet over medium-high heat and add a tablespoon of oil. Once shimmering, add the steak chunks in a single layer, searing until browned on all sides, about 3-4 minutes. Remove the steak and set aside.
Add the minced garlic and butter to the same skillet; cook over medium-low heat until the butter melts and the garlic becomes fragrant, about 1 minute. Stir well to combine.
Return the seared steak to the skillet, toss to coat in garlic butter, and cook for another 1-2 minutes until heated through and well coated.
Meanwhile, mash the cooked potatoes using a potato masher or fork. Mix in warm milk gradually until the mash is smooth and creamy. Season with salt and pepper to taste.
Serve the hot steak bites alongside a generous scoop of creamy mashed potatoes. Drizzle any remaining garlic butter from the skillet over the steak for extra flavor.