Wash the potatoes and wrap each in plastic wrap. Microwave for 2 minutes, then let them steam for 5 minutes. Remove the wrap, peel the potatoes, and mash them roughly with a fork in a bowl.

Season the mashed potatoes with salt and black pepper, mixing well.

Slice the onion thinly along the grain. Trim the thick stems of the watercress and cut into 2cm lengths.

Heat salad oil in a frying pan over medium heat. Add the beef and stir-fry briefly until the color changes. Add the onion and salt-pepper seasoning, stir-frying until the onion softens. Transfer the mixture to the bowl with the potatoes and mix well.