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Simmer the onions in the stock for 30 mins, adding a little water if needed to keep them covered. Meanwhile, in a small pan, melt the butter, then add the flour and cook for 1-2 mins. Stirring constantly, gradually pour in the milk, then the cream, and bring to the boil. When you have a smooth white sauce, remove from the heat. Allow to cool a little before stirring in the egg yolks, spices and seasoning.
Heat grill to high. Drain the onions and lay them in a single layer in an ovenproof dish. Spoon over the cream sauce to cover. Grill for 5 mins until lightly browned, then scatter over the basil before serving.