If you cook regularly—and especially if you use chef recipes—you will inevitably come across the monter au beurre. The French term translates to "mount with butter" or in everyday terms, to finish a sauce with butter. Using this technique takes any plain sauce or gravy and makes it sparkle (look at the image on this page and you will see how the sauce shines). It also adds a lovely soft viscosity to the sauce as well as some additional flavor. Chefs love this technique because the finished sauce looks lovely and especially appealing when poured over or served with their dish.
The technique is simple enough, but there are a few pitfalls easily avoided, as outlined below. Avoid these, and you will be using beurre monte to finish all of your sauces.
For two cups of sauce or gravy, use no more than 1/2 ounce of butter. Cut the butter into small cubes and place into the freezer for 30 minutes to chill it right down. Never use warm butter as this will not incorporate into the sauce.
Heat the sauce to no higher than 195 F and—most importantly—never boil the sauce. This will be too hot for the butter, which will melt far too quickly and pool into the sauce, making it impossible to incorporate.
Take the butter from the fridge, remove the sauce from the heat, check the temperature if you can, and add one small piece of butter to the sauce. Then, either shake the pan to create small waves of sauce or gently whisk to scoop up the butter, and with continued shaking or whisking, you will see the butter disappear into the sauce. The sauce will slowly start to thicken slightly and take on a lovely shine. Continue until you have the shine you want, remembering never to use more than 1/2 ounce to 2 cups ratio. If at any point the butter struggles to blend in, stop.
How to Reheat the Finished Sauce
Once the sauce is ready, it can be gently re-warmed or held until ready to serve by placing over a pan of gently simmering water (a bain-marie). Never allowing the water to boil or touch the sauce. A gentle shake from time to time will help prevent any skin from forming on the surface.
Any leftover sauce that has been made using a beurre monter finishing should be stored well covered in the refrigerator. It will keep well for a couple of days, but not longer.
If you wish to reheat a sauce that has been refrigerated, gentleness is the key. You cannot rush the reheating. The best way to ensure the sauce does not split is to do the bain-marie method above. Never use a microwave.
What to Do If Your Sauce Splits
Because beurre monter is an emulsion, it can be delicate. If a sauce with beurre monter is heated too quickly, it can often split. If you work quickly, this can be rescued firstly by adding a few teaspoons of iced water and whisking vigorously. The coldness will reset (emulsify) the butter and it should come back together. If that fails, try whisking in a little more ice-cold butter, though adding too much can also dramatically alter the flavor and turn it into a butter sauce which is not what you want.