Boil the cubed potatoes in salted water until tender, about 15 minutes. Drain and mash with butter, milk, salt, and pepper.
Heat olive oil in a skillet over medium-high heat. Add steak pieces, season with salt, pepper, paprika, and red pepper flakes, and sear for 2-3 minutes per side.
Reduce heat to medium, add butter and minced garlic, and stir until the butter is melted and the garlic is fragrant.
Remove from heat, garnish with chopped parsley, and serve hot with the mashed potatoes.