Pop the meat in the freezer for a few minutes to stiffen it up for slicing.
Place potatoes in a pot and cover with cold water. Bring water to a boil. Salt the water and cook the potatoes until tender, 12 to 15 minutes. Drain. Halve the steak lengthwise then very thinly slice the meat. Heat the oil in a very large skillet over high heat. When oil wafts smoke and ripples, add the meat and stir-fry until caramelized 3 minutes. Add peppers and stir-fry 2 minutes then add in scallions and edamame for 2 minutes more. Stir together the sugar, soy, mustard and ketchup and pour over stir-fry the last minute of cooking. Season, to taste, if necessary. Mash potatoes with buttermilk, salt and pepper, hot sauce, parsley, dill, chives, adjust seasoning with salt and pepper, to taste, and serve with stir-fry on top!;