Best Chicken Pot Pie Casserole

Ingredients

• 4 tbsp. butter
• 3 medium carrots, diced (about 1 1/2 cups)
• 3 stalks celery, diced (about 1 1/2 cups)
• 1 large yellow onion, diced (about 1½ cups)
• 1 tsp. chopped fresh thyme 
• 1 tsp. kosher salt, divided
• 1/2 tsp. freshly ground black pepper
• 4 tbsp. all-purpose flour
• 2 c. whole milk
• 1 c. low-sodium chicken broth
• 1 tbsp. Dijon mustard
• 1/2 tsp. garlic powder
• 4 c. cooked shredded chicken
• 1 c. frozen peas
• 1 (16.3-ounce tube) biscuit dough