Boil potatoes: Add diced potatoes to a pot of water, boil 15 min, then drain.
Simultaneously cook the chicken: Melt 2 Tbsp butter in a pan over medium-low. Add chicken, cook until no pink remains (2–3 min). Remove from pan.
Sweat veggies: Melt 2 more Tbsp butter. Add onion, carrot, celery. Cook until onions are translucent (about 5 min).
Build the sauce: Stir in flour, then add thyme, sage, cooked potatoes, cooked chicken, and stock. Simmer until thickened to your liking. Too thin? Add a quick slurry: 1 part water : 1 part cornstarch or flour.
Finish filling: Stir in peas and optional cream. Remove from heat.
Biscuit topping: Prepare biscuit mix per box instructions. Top each portion of filling with a ½-inch layer of dough (No need to spread to the edges… it puffs up).
Bake: Bake as per box instructions until golden brown. Brush with the remaining 4 Tbsp butter (use the garlic-spiced butter if using Cheddar Bay mix).
