Place the chicken breasts and bay leaf in a saucepan and cover with cold water by 1 inch. Add a generous pinch of salt.

Bring to a gentle simmer over medium heat, then lower the heat so it barely bubbles. Cook 12 to 15 minutes, until the thickest part reaches 165°F. Let the chicken cool in the liquid for 10 minutes.

Meanwhile, toast the pecans in a dry skillet over medium heat for 4 to 5 minutes, shaking often, until fragrant. Cool and roughly chop.

Finely dice the celery. In a large bowl, whisk the mayonnaise, sour cream, Dijon, lemon juice, salt, and pepper.

Shred the chicken with two forks and add it to the bowl with the celery, green onion, and most of the pecans. Fold until evenly coated.

Taste and adjust the seasoning. Top with the remaining pecans and serve, or chill for at least 30 minutes to let the flavors come together.