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Prick the potatoes all over with a fork, then put in a microwaveable bowl, cover and microwave on high for 10 mins or until soft in the centre. Meanwhile, heat the oil in a frying pan over a medium heat, add the shallots and cook, stirring often, for 10 mins or until soft and beginning to brown. Remove and set aside.
Cook the broad beans in a pan of boiling salted water for 3 mins, then drain and run under cold water. Pop half out of their pale thicker skins and leave the rest.
To assemble, put a tortilla in the same frying pan you used for the shallots, over a medium heat. Scatter over a quarter of the cheese, then squeeze over half the potatoes, skin included, sprinkle over half the onions and broad beans, and finish with another quarter of the cheese. Season well, then put a second tortilla on top.
Leave to cook and heat through for 2 mins, then flip over to cook the other side for another 2 mins. Tip out onto a board and repeat with the remaining ingredients. Slice and serve.