Beurre rouge is a simple emulsified butter sauce that's great with fish or seafood. If you've heard of beurre blanc, it's the same, only beurre rouge (literally "red butter" in French) is made with a reduction of red wine instead of white.
It's a nice sauce to have in your repertoire because you can whip up a batch on the spot (all you really need is red wine and butter), making it ideal for emergencies. But to do it properly, you'll want to include the shallots and vinegar as well. The shallots in particular add an intangible but still potent flavor and aroma to the sauce; while the vinegar imparts a concentrated tartness that helps cut through the richness of the butter.
Like beurre blanc, you can serve beurre rouge as an accompaniment with scallops, lobster or other fish or seafood. But because it's richer than beurre blanc, you can even serve it with a steak.
Good wines for the reduction (or au sec, meaning "nearly dry") include cabernet sauvignon, pinot noir or chianti, but any drinkable dry red will do. As for the red wine vinegar, there's no need to go crazy. The everyday stuff you keep in your pantry for making salad dressing will be fine.
Important: Your butter needs to be very cold. Otherwise, the emulsion will break and instead of a smooth, velvety sauce, you'll end up with what looks like a pot of red wine with melted butter on top. This can also happen if you add the butter cubes too quickly, or don't whisk hard enough. If that happens, take it off the heat and whisk in a few chips of ice until the emulsion comes back together.
- Heat the wine, vinegar, and shallots in a saucepan until the liquid comes to a boil, then lower the heat a bit and continue simmering until the liquid has reduced down to about 2 tablespoons. This should take about 10 minutes.
- While the liquid reduces you can cut the butter into medium (1/2-inch) cubes, but either leave this until the reduction is nearly finished or return the butter cubes to the refrigerator to keep them cold while the liquid finishes reducing.
- Once the wine-vinegar mixture has reduced to 2 tablespoons, reduce the heat to low and start adding the cubes of butter, one or two at a time, and whisk rapidly with a wire whisk.
- As the butter melts and incorporates, add more butter and keep whisking. Continue until you only have 2 to 3 cubes remaining. Remove from heat while whisking in the last few cubes, and whisk for a moment or two more. The finished sauce should be thick and smooth.
- Season to taste with Kosher salt. Traditionally you would strain out the shallots before serving, but you don't have to do this. Do serve it right away, however.