Chicken Pot Pie Casserole

Ingredients

• 1/4 c. butter
• 1 small yellow onion, chopped small (about 1 c.)
• 1/2 c. celery, sliced very thin, from about 2 ribs
• 1/3 c. all-purpose flour
• 1 1/2 c. chicken broth
• 1 c. milk
• 1/2 t. kosher salt
• 1/4 t. black pepper
• 1/4 t. celery seed
• 4 c. cooked chicken, diced into 1/2 inch pieces (rotisserie chicken is great)
• 2 c. frozen mixed vegetables
• 2 c. all-purpose flour
• 2 1/2 t. baking powder
• 1 t. kosher salt
• 1 T. sugar
• 1/3 c. cold butter, grated
• 1 c. milk, plus more ONLY as needed