Asparagus cooked sous vide is similar to blanched asparagus but ends up with a stronger flavor and slightly firmer texture. Because asparagus is so tender to start with you only need a short cooking time, usually 10 to 30 minutes, depending on the thickness of the asparagus. For thicker asparagus it can be helpful to peel off the tougher outer layer and they might need a little longer in the water bath.
Asparagus and Dijon mustard pair really well so I make a simple Dijon vinaigrette to dress the finished asparagus with. It adds tangy base flavors while not overpowering the asparagus itself.
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Preheat the water bath to 183°F (83.9°C).
Place the asparagus in a sous vide bag, trying to keep the thickness of the bag less than 1" (25mm) for even cooking. Salt and pepper the asparagus then seal the bag. Place in the water bath and cook for 10 to 30 minutes.
Once the asparagus is tender remove it from the bag.
Combine all the ingredients and whisk or blend together. The vinaigrette can be made several hours ahead of time and re-whisked just before serving.
Place the asparagus on a plate and drizzle the Dijon mustard vinaigrette over the top. Sprinkle the tarragon on top and serve.
I almost always thicken the vinaigrette by blending in 0.2% to 0.3% xanthan gum. It really clings to the asparagus a lot better with the addition of the xanthan gum.