Boil the sliced potatoes for 10 minutes until just tender, drain and set aside.
Meanwhile, heat a large frying pan (about 22cm) with a heatproof handle and dry-fry the frozen veg for 5-10 minutes, until cooked through and any liquid has evaporated. Set aside.
In a bowl, whisk the eggs with the milk and half the grated cheese. Tip in the potatoes and veg, and stir to combine. Heat the oil in the pan and pour in the frittata mixture. Scatter the remaining cheese over the top and cook over a medium-low heat for 15 minutes. About 5 minutes before it’s done, preheat the grill to hot.
Place under the grill for 5 minutes, until golden on top and set and cooked through. Serve with the salad.