Place the peeled and cubed potatoes into a large pot and cover with cold water. Add a pinch of salt, then bring to a boil over medium-high heat. Reduce the heat and simmer gently until the potatoes are tender when pierced with a fork, approximately 15 minutes.

Drain the cooked potatoes and return them to the warm pot. Add butter and milk, then mash until smooth and creamy. Adjust the texture with a splash of milk if necessary, and season with salt and pepper to taste. Cover to keep warm while preparing the steak.

In a large skillet, heat the vegetable oil over medium-high heat until shimmering. Add the steak pieces in a single layer, ensuring they are not overcrowded. Sear each side for about 2–3 minutes, or until a rich golden crust forms. Work in batches if required.

Reduce the heat slightly and add the minced garlic, butter, thyme, and rosemary to the pan. Stir continuously as the butter melts, coating the steak pieces with the fragrant mixture. Allow the flavors to meld for an additional minute, then remove from heat.

Spoon a generous portion of the creamy mashed potatoes onto each plate, top with the garlic butter steak bites, and drizzle the remaining pan sauce over the top. Serve immediately while hot for optimal flavor and texture.