Put each fillet in a ziploc bag with a couple of slices of lemon and 2-3 tablespoons of vinaigrette.
Submerge in cold water to squeeze out all the air, then place in the heated water for 30 minutes at 135 degrees.
For the Beurre Blanc
Cook the shallot, wine and vinegar until the liquid is reduced to just 1-2 tablespoons. If you go too far, add in a tablespoon of water.
Once it’s reduced add the tablespoon of cream.
Reduce the heat to low, then whisk in the chilled butter pieces one at a time. Keep the butter moving so it emulsifies rather than just melting. [Take the pan off the heat if it’s getting too hot].
Once all the butter is whisked into the sauce season with salt and pepper, and stir in the drained capers.
Recipe by From Here to There at https://blog.garritys.org/2014/01/sous-vide-dabs.html