Hairy Bikers Chicken Stew and Dumplings

Name: Hairy Bikers Chicken Stew and Dumplings
Ingredients: 720g chicken thigh fillets, quartered; 3 carrots, peeled and chopped; 3 medium-sized potatoes, peeled and chopped; 2 celery sticks, sliced; ½ tsp salt; ½ tsp black pepper; 1 tsp dried thyme; 1.2 liters chicken stock; 45g unsalted butter; 1 large onion, finely chopped; 50g plain flour; 240g plain flour (for dumplings); 3 tsp baking powder; 100g grated strong cheddar; ½ tsp dried parsley; 2 tbsp unsalted butter, melted (for dumplings); 180ml milk