Place the peeled and chunked potatoes into a large pot and cover with cold water. Add 1/2 teaspoon salt to the water.
Bring the pot to a boil over high heat and cook the potatoes until tender, about 15 minutes. Drain and return potatoes to the pot.
While the potatoes cook, season the steak cubes with 1 teaspoon salt and 1/2 teaspoon black pepper.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat until shimmering.
Add the steak bites to the skillet in a single layer, cooking in batches if necessary to avoid overcrowding. Cook for 2-3 minutes on each side until browned and cooked to desired doneness. Remove steak bites from skillet and set aside.
Reduce heat to medium and add 2 tablespoons of butter to the same skillet. Once melted, add the minced garlic and cook for 1 minute until fragrant, stirring frequently to avoid burning.
Return the steak bites to the skillet and toss in the garlic butter until well coated. Remove from heat.
Mash the drained potatoes with a potato masher or fork until smooth.
Add the remaining 2 tablespoons butter, whole milk, and sour cream to the mashed potatoes. Stir until creamy and well combined.
Season the mashed potatoes with 1/2 teaspoon salt and 1/4 teaspoon black pepper, adjusting to taste.
Divide the mashed potatoes among plates, top with garlic butter steak bites, and garnish with chopped fresh parsley.