In a medium saucepan, combine potatoes and peeled garlic cloves. Cover with water and bring to boil. Reduce to a simmer and cook until potatoes are tender, 15 to 20 minutes.
Meanwhile, in a food processor, combine parsley, cilantro, shallot, garlic, oil, vinegar, and red pepper flakes and process until they form a well-combined sauce. Transfer chimichurri sauce to a bowl.
Preheat the broiler. Score steak diagonally on both sides. Season both sides with salt and black pepper to taste. Broil 3 to 4 minutes on each side. Let sit for 5 minutes before thinly slicing across the grain.
When potatoes are tender, carefully drain and return to the pan. Place the pan over low heat and add the milk. Mash together until potatoes reach desired texture and consistency. Season with salt and black pepper.
Arrange steak slices on a serving platter. Pour chimichurri over steak slices and serve alongside mashed potatoes.