Preheat the oven to 180C.

Scrub the new potatoes and cut into 4-5mm slices. Peel the onion and cut into wedges.

Heat half the olive oil in a large, heavy-based skillet that can go in the oven (i.e. no plastic handles etc). Cook the potatoes and onions for 8-10 minutes until the onions are soft and the potatoes are starting to brown. Remove from the pan and set aside.

Slice the zucchini into 6-8mm slices. Heat the remaining oil and cook the zucchini slices for 3-4 minutes until starting to brown.

Return the potatoes to the pan, and mix gently to avoid breaking up the vegetable slices. Cook for a few minutes, stirring occasionally to prevent sticking, until the potatoes are warmed through.

Beat the eggs and milk together. Pour the egg mixture into the pan - it should just cover the vegetables. Cook on a very low heat for 5-6 minutes, then transfer to the oven and continue to cook for a further 10-15 minutes, until the eggs are set.

Remove from the oven, run a knife around the edge of the tortilla, and invert onto a serving plate. Cut into wedges and serve.