A very useful thing to have around the kitchen, beurre monté is basically emulsified water and butter. I use it place of cream in a lot of my sauces because I think cream masks the flavors of other ingredients. The long-cooked shallots help keep the sauce from separating. I usually wind up using about 6 cups water. The sauce will keep, covered, in the refrigerator, for more than a week, or in the freezer indefinitely. If the sauce breaks when you reheat it, throw it back in the blender or use an immersion blender to re-emulsify it.
YIELD: 4 CUPS
8 large shallots, ends trimmed, thinly sliced
4 cups water, plus more if needed
1½ pounds unsalted butter, cut into chunks
In a large saucepan, combine the shallots and water over medium heat and bring to a simmer. Lower the heat and cook until the shallots are literally falling apart, 45–60 minutes. If you need to add more water as the shallots cook, do; in the end, you want as much water as shallots in the pot.
Put the shallots and remaining water in a blender. Holding the cover of the blender with a towel, blend, starting on the lowest speed and gradually increasing it to high until you have a smooth purée. With the blender running, add the butter, a little at a time, and blend until you have an opaque, creamy sauce that coats the back of a spoon.