Creamy Chicken with Cheddar Biscuits

• FOR FILLING:
• 1 1/2 cups milk
• 2 tbsp chopped fresh thyme
• 1 tsp salt
• 4 skinless, boneless chicken breasts cut into bite-sized pieces, about 1 kg
• 1 tbsp vegetable oil
• 1 small sweet potato, peeled and cut into small cubes
• 1 celery stalk, thinly sliced
• 1/2 red pepper, chopped
• 1/2 onion, chopped
• 3 tbsp butter
• 1/4 cup all-purpose flour
• 1/2 cup frozen peas

• FOR CRUST:

• 1 1/4 cups all-purpose flour
• 2 tsp baking powder
• 2 tsp granulated sugar
• 3/4 tsp salt
• 3 tbsp cold unsalted butter, cubed
• 3/4 cup grated old cheddar
• 1 egg
• 1/4 cup milk

1. Preheat oven to 375F. Heat 1 1/2 cups milk with thyme and 1 tsp salt in a large frying pan over medium-high. When simmering, add chicken. Cook, stirring often, until firm, about 6 min. Transfer chicken to a platter. Pour liquid out of pan and set aside.
2. Return pan to burner. Add oil, then sweet potato, celery, pepper and onion. Stir often until vegetables begin to soften, 2 to 3 min. Transfer vegetables to same platter as chicken.
3. Add butter to same pan. Gradually whisk in 1/4 cup flour and cook 1 min. Gradually whisk in poaching liquid. Whisk until sauce is smooth and thickened, 2 to 3 min. Stir in peas, chicken and vegetable mixture. Season with pepper. Stir to combine. Transfer filling into an 8-in.-square baking dish and place on a rimmed baking sheet. Set aside.
4. Stir 1 1/4 cups flour with baking powder, sugar and 3/4 tsp salt in a large bowl. Using the back of a fork, mash in butter until crumbly. Stir in cheese. Whisk egg with 1/4 cup milk in a medium bowl. Pour egg mixture into flour and stir until just moistened. Turn out dough onto a floured counter and gently knead until it comes together. Shape into a log about 9 in. long and 2 1/2 in. wide. Slice into 9 rounds. Lay on top of the chicken mixture. Brush tops of dough with more milk.
5. Bake in centre of oven until biscuit tops are dark golden and filling is bubbly, about 25 min. Let stand 10 min before serving.