Dice the red onion into 1/4-inch pieces, dice the celery into similar-sized pieces, finely chop the tarragon, parsley, and chives, mince the garlic clove, and zest the lemon before cutting it in half for juice. Season the 2 lb chicken generously with salt and pepper on all sides. This prep work ensures everything is ready when you need it and prevents scrambling during assembly.

Place the seasoned chicken in a vacuum-seal bag along with the lemon slices (1/4-inch rounds) and tarragon sprigs. Seal the bag, removing as much air as possible. Cook in a water bath at 155°F for 1 to 4 hours depending on your schedule—the gentle temperature ensures incredibly moist, tender meat. Once cooked, immediately transfer the bag to an ice bath for 15 minutes to stop the cooking process and cool the chicken slightly for handling.

While the chicken cools, combine the mayonnaise, Dijon mustard, lemon juice and zest in a large bowl. Stir in the minced garlic, smoked paprika, onion powder, diced red onion, diced celery, and the chopped tarragon, parsley, and chives. Mix everything thoroughly so the flavors meld together—I like to let this sit for a minute so the spices fully dissolve into the mayo and the vegetables start releasing their juices. This creates a more cohesive, flavorful base for the chicken.

Remove the cooled chicken from the bag and discard the lemon slices and tarragon sprigs. Carefully remove the skin and bones, then cut the chicken into 3/4-inch cubes—this size is ideal for a hearty salad that holds together well on bread or lettuce. Add the cubed chicken directly into the bowl with the salad base and fold gently to combine, being careful not to break up the chicken pieces. Taste and adjust the seasoning with salt and pepper to your preference—I find that the chicken often needs a bit more salt than you'd expect since it was poached gently.

Serve the chicken salad immediately on fresh bread, crisp lettuce leaves, or however you prefer. The salad is best served while the chicken is still slightly cool from the ice bath, which provides a nice contrast to warm bread if you choose to toast it.