In the bottom of a saucepan spread the finely minced onions, and put the Brussels sprouts on top of them.
Moisten with stock, put on the lid and cook for 1 hour over a moderate heat, without mixing the two sorts of vegetable together.
When they are tender, take out.
most of the Brussels sprouts.
Make a puree of the rest together with the onions—by mashing through a sieve, or using a food mill.
If necessary, add a little stock to the puree to make a smooth sauce of it, into which you stir the Brussels sprouts.
Add plenty of pepper, and sprinkle with chopped parsley.
This dish, you see, contains no fats whatever.
If you do want to add some butter, then let it melt into the sauce containing the sprouts, after you have moved the dish away from the heat.