Made from chunks of butter and just a few drops of water whisked over moderate heat, Beurre Monté is considered a "workhorse sauce," as Thomas Keller calls it. Used for basting meats, poaching lobster, and sauteeing any number of foods, Beurre Monté helps keep foods moist, enhances flavors, and helps cook foods, as the butter layered evenly helps distribute heat.
Heat one tablespoon of water until boiling in an appropriately-sized pan. Reduce the heat to low, and begin whisking in chunks of butter to emulsify. Keep the heat low and constant. Keep sauce warm until ready to use. Any unused portions can be refridgerated and reheated later.
Easy enough, right?
Speaking of Thomas Keller, here is a recipe from his The French Laundry Cookbook.
Sweet Potato Agnolotti with Sage Cream, Brown Butter, and Prosciutto
Sweet potato filling
1 1/2 pounds sweet potatoes
4 oz. unsalted butter
2 slices bacon, cooked, cut into 1/4-inch dice
Pinch of nutmeg or allspice
Ground black pepper
Fresh pasta dough
Preheat oven to 350F.
Bake potatoes in tinfoil until soft, 1-2 hours, depending on size of potato. Skin potatoes and discard. Push potato through ricer and place in saucepan. Add bacon. Stir over low heat, seasoning with nutmeg or allspice and salt and pepper to taste. Mix in 4 tbsp. butter. Yields filling for 48 agnolotti. Roll out the dough, and make agnolotti using filling.
1/3 cup sage leaves
1 cup crème fraîche
1 cup beurre monte
Blanch sage leaves in boiling water for 2 minutes. Drain in cool water, and drain again. Squeeze dry.
Heat the crème fraîche and beurre monte with the salt on low heat; do not boil. Place the sage in a blender and process to chop it. With the motor running, pour the hot cream mixture through the top and blend thoroughly. Strain, salt and pepper to taste, and set aside.
In a small saucepan, heat oil for deep-frying, and lightly fry the sage leaves. Dry on paper towels and set aside.
Place the butter in a skillet over medium heat and cook until brown. Reduce heat and keep warm.
Cook the agnolotti. Drain and mix with sage cream.
Set agnolotti in dish with cream sauce. Sprinkle with prosciutto, and garnish with fried sage leaves.