Fill a pot with water, add the cubed potatoes, and bring to a boil. Cook until the potatoes are tender, about 15 minutes. Drain the water and return the potatoes to the pot.
In the pot with the cooked potatoes, add 4 tablespoons of butter, ½ cup of milk, 1 teaspoon salt, and ¼ teaspoon black pepper. Mash the potatoes until smooth and creamy. Set aside and keep warm.
While the potatoes are cooking, heat a large skillet over medium-high heat and add 2 tablespoons of butter.
Season the steak bites with salt, black pepper, and fresh thyme.
Add the steak bites to the skillet in a single layer. Sear the steak for about 2-3 minutes on each side or until browned and cooked to your desired doneness. Remove from the skillet and set aside.
In the same skillet, lower the heat and add the remaining 2 tablespoons of butter and minced garlic. Cook for about 1 minute until fragrant, being careful not to burn the garlic.
Return the steak bites to the skillet and toss them in the garlic butter until well coated. Cook for an additional minute to combine the flavors.
To serve, place a generous scoop of mashed potatoes on a plate, top with garlic butter steak bites, and garnish with chopped parsley.