In a large frying pan, cook the onions in the olive oil over medium heat until soft.
Line a 25cm non-stick flan tin with the shortcrust pastry sheet. Trim the edge.
In a large bowl, add the cooked onions, then the eggs and the grated cheese. Stir well and season with salt and pepper. Pour the onion mixture into the pastry case, spreading it evenly.
Cook in the preheated oven for 30 minutes or until the edges of the tart start to brown. Serve hot or cold.