Boil the potatoes, peel them and grate them using a large-hole grater. Chop the onion and grate the cheese with the same grater. Place the ingredients in a bowl, adding salt and abundant pepper, then blend the ingredients homogeneously. Divide the mixture in four parts and shape as many meatballs. Then squash them. Heat two spoons of extra virgin olive oil into a pan and there cook the meatballs until golden-brown on both sides, then dry them with kitchen paper. Finally, salt them slightly. Julienne the courgettes, peel the pepper after toasting the thin skin covering it, halve it, scoop the seeds and the white ribs out and cut it like the courgettes. Thinly slice the spring onions. Stir-fry the vegetables in a pan with the remaining oil and lightly salt them. Serve the rosti, hot, in single plates, after topping them with sautéed vegetables.