Nice big onions are peeled, cut into fine slices and then stewed in boiling hot salted water until half soft. Later, they are poured into a sieve to drain. In the meantime, melt the fat, add the drained onions and cook until soft. Later, make a light roux, pour in milk, add onions and season well with salt and pepper. Finally, pass everything through a fine sieve or machine and heat the puree repeatedly.
It tastes very fine with duck, goose or possibly mutton chops.
A goose becomes especially tender when roasted at low temperature (approx. 80 °C). But then increase the roasting time to at least 6 hours!