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.