In a pot, bring water to a boil. Add the cubed potatoes and cook until tender, about 15 minutes. Drain and return to the pot. Add the butter, milk, salt, and pepper. Mash until smooth and creamy. Set aside and keep warm.
In a saucepan, bring water to a boil. Add the green beans and cook for about 4-5 minutes until bright green and tender-crisp. Drain and toss with olive oil, salt, and pepper. Set aside.
Heat a large skillet over medium-high heat. Add 2 tablespoons of butter and allow it to melt. Season the steak bites with salt and pepper. In batches, add the steak bites to the skillet, ensuring not to overcrowd, and cook for about 2-3 minutes until browned. Flip and cook for an additional 2-3 minutes for desired doneness. Remove the steak bites and set aside.
In the same skillet, melt the remaining 2 tablespoons of butter. Add the minced garlic and sauté for about 30 seconds until fragrant. Return the cooked steak bites to the skillet and toss to coat in the garlic butter sauce.
On each plate, serve a generous portion of mashed potatoes, a side of green beans, and top with the garlic butter steak bites. Garnish with fresh parsley if desired.