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["Preheat oven to 400 deg F/200 deg Celsius Have ready a tart plate or dish, not too large.", "Peel the onions and cut into thickish rounds, about 1/4 inch thick. It's not too important, but the rounds should not be too thick to soften properly.", "Heat the olive oil in a pan to quite a high heat. Add the onions. They'll break apart -- no worries. Let them fry over high heat for a few minutes. Then sprinkle over the balsamic vinegar, and top with the brown sugar. (Any brown sugar will do).", "Before you turn down the heat, turn over the onions once. Then lower heat, put on a lid, and let stew slowly for about 15 - 20 minutes. If, after that time, you find some liquid in the pan, remove the lid, turn up the heat, and let the onions simmer until the liquid goes gooey.", "Add the mushrooms, stir in, and let fry for about 5 minutes. (You can also use a can of whole mushrooms, of about 15 - 16 oz. Drained, that will yield about 7 - 8 oz as well). Add the walnuts, stir through.", "Sprinkle very lightly with sea salt (the blue cheese is also salty). Stir in the walnuts, and pull off the heat.", "Roll out the pastry, and cut out a round about 1 inch (2 cm) larger than the top of the tart plate or dish. Beat the egg.", "Line the bottom of a tart plate or tin (not loose-bottomed) with a round of baking paper. Tip in the contents of the pan, with all juices, into the bottom of the tart plate. Crumble the blue cheese on top.", "Cover the top of this lot with the pastry ""lid"
