Peel and slice 2 medium onions (1 pound/500 grams), and blanch them for 10 minutes in boiling salted water. Drain the onion slices and gently cook them in butter until they are completely soft, usually about 20 minutes. Do not allow them to brown. Add 2 cups (500 milliliters) sauce béchamel to the onions, and season with salt, white pepper, and a pinch of sugar. Cover the pot or saucepan and put it in a 300°F (150°C) oven for 1 hour. Strain the sauce and finish it with ½ cup (125 milliliters) heavy cream and 1 ounce (30 grams) butter.