Slice each chicken breast horizontally to create thinner, even pieces.
Dice onion and celery into small pieces.
Melt butter in a small bowl and measure out chicken broth.
Arrange sliced chicken on the bottom of the crockpot. Season with salt and pepper, then scatter onion and celery on top.
Pour 2 cups chicken broth over the chicken layer.
Sprinkle dry stuffing mix evenly over the broth, then drizzle melted butter across the top.
Cover and cook on low for 4 to 5 hours.
Check chicken by piercing the thickest piece; it should shred easily and show no pink inside.
Gently stir with a wooden spoon to combine stuffing with broth, breaking up clumps without shredding the chicken.
Taste and adjust seasoning if needed, then serve straight from the crockpot into bowls.