How to Make Beurre Fondue
Beurre fondue is a butter and water emulsion similar to beurre monté or beurre blanc, usually used as a poaching liquid instead of a sauce. Though very rich, beurre fondue takes on flavor well (including fat-soluble flavors that might not otherwise be extracted) and passes those flavors to additional things poached in it. For example, try poaching wild ramps (or substitute any kind of onion) or fresh wild mushrooms in the beurre fondue first, then pull out the ramps and poach halibut cheeks in the same beurre fondue.
Put about ½ inch of water in a pan and heat it to a simmer. Keeping the liquid at a simmer, slowly whisk in butter, 1 tablespoon at a time. (We used 1 ½ sticks here). Heat control is very important. You want the butter to melt gently into the water. If the liquid becomes too hot, it can “break” (the separation of an emulsion into its component parts). Err on the side of having the emulsion too cool and reduce the heat as you add more butter.
If the mixture breaks (and starts looking oily rather than smooth), try adding a little more water and whisking vigorously. There is a chance that this will allow you to rebuild the emulsion without having to start over. Keep the finished beurre fondue warm until ready to use.