Cook the Chicken – In a large skillet over medium heat, melt 1 tablespoon butter. Add diced chicken, season lightly with salt and pepper, and cook for 6–8 minutes, stirring occasionally, until cooked through and no longer pink in the center. Remove from heat.
Preheat Oven – Preheat your oven to 375°F (190°C).
Mix the Filling – In a large bowl, combine cooked chicken, celery, water chestnuts, cream of chicken soup, mayonnaise, and sour cream. Stir until evenly mixed.
Bake Base Layer – Lightly coat a 9×13-inch baking dish with cooking spray. Spread the chicken mixture evenly into the dish. Bake for 25–30 minutes, or until celery is crisp-tender.
Add Croissant Layer – Remove casserole from the oven. Unroll the croissant dough into flat pieces and arrange evenly over the hot filling.
Brush & Bake – Lightly brush dough with melted butter. Return to oven and bake for 10–12 minutes, or until golden brown.
Top with Cheese – Remove from oven, sprinkle Parmesan cheese over the croissant topping, and return to oven for 1–2 minutes to melt the cheese.
Rest & Serve – Let stand 5 minutes before serving.