Onion Sauce (Called also Soubise Sauce)

A plain white roux sauce enriched with cooked onion and cream. The one thing worth deciding before you start is how smooth you want it: classic soubise is sieved or pureed, while this version leaves the onion in slices.

• Texture is the hingeNamed soubise sauces are normally passed through a sieve or blended so the onion disappears into a pale, velvety sauce. As written the slices stay whole, which gives something closer to a British onion sauce for mutton or roast lamb. Either is correct; choose one and treat the onion accordingly.
• The rouxTwo tablespoons each of butter and flour cooked "until smooth" means a white roux: two to three minutes over moderate heat, foaming but not colouring. Any browning will tint the finished sauce.
• Hot stock, added slowlyOne cup of hot stock into that quantity of roux gives a thick sauce, and the cream thins it later. Add it in a few stages, whisking each addition smooth before the next, to avoid lumps.
• Stock choiceThe recipe specifies only 'stock'; a light veal or chicken stock suits it. A low-sodium chicken stock is the closest supermarket equivalent; hold back on salt until the cream is in and you can taste the finished sauce.
• Cooking the onionsFifteen minutes in salted water is right for sliced onions of moderate thickness, and it is there to remove harshness rather than to brown them. Drain them well, or the sauce will be watery.
• Cream at the endHalf a cup of hot cream goes in off the boil or at the barest simmer. Heating the cream first prevents the sauce breaking temperature and thinning unevenly; do not boil hard after it is in.
• CayenneCayenne here is a seasoning, not a heat element: a scant pinch, enough to sharpen the onion without showing red flecks in a white sauce.
• Yield and servingMakes roughly two cups with the onions in, enough for four to six servings. It suits roast or boiled lamb, poached chicken, eggs, or plain white fish.