Put the potatoes in a pan of cold water, bring to the boil, then simmer for 10-15 minutes. Remove from the heat, drain, then run under cold water until cool enough to handle. Cut into chunks and put in a large bowl.
Meanwhile, heat half a tablespoon of the olive oil in a 23cm frying pan and cook the bacon for 3-4 minutes. Add the onion and garlic and cook for a further 5 minutes. Put into the bowl with the potato chunks. Grate in the cheese and add the eggs and half the watercress. Mix to combine and season with black pepper.
Heat the grill to high. Heat the remaining oil in the same frying pan, then pour in the frittata mixture and cook over a low heat for 15-20 minutes. Put under the grill for a further 5-10 minutes until fully set, then serve sliced with the remaining watercress and bistro salad.