1) Place meat in freezer for a few minutes to harden it for slicing.
2) Place potatoes in a pot and cover with cold water. Bring water to the boil. Salt the water and cook the potatoes until tender, 12 to 15 minutes. Drain.
3) 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 caramelised 3 minutes.
4) Add peppers and stir-fry 2 minutes then add in green onions, and edamame for 2 minutes more. Stir together the sugar, soy, mustard and tomato ketchup and pour over stir-fry the last minute of cooking. Season, to taste, if necessary.
5) 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!