If using raw chicken, poach or bake until cooked through, then let cool and shred or dice into bite-sized pieces.
In a large bowl, whisk together mayonnaise, sweet pickle relish, yellow mustard, and lemon juice until smooth.
Add the chicken, celery, red onion, toasted nuts, dill, parsley, and chives to the bowl.
Gently fold everything together until evenly coated. Be careful not to overmix—you want some texture.
Season with salt and pepper. Taste and adjust if needed.
Cover the bowl and refrigerate for at least 30 minutes (or up to overnight) to let the flavors meld.
Serve chilled or at room temperature. Garnish with extra fresh herbs if desired.