Chicken Poppy Seed Casserole

4 tablespoons butter, plus 1 tablespoon to grease
1 tablespoon olive oil
1 onion, small dice
2 carrots, peeled, small dice
3 stalks celery, small dice
1/4 teaspoon cayenne pepper
2 teaspoons fresh thyme leaves
1/4 cup flour
2 cups chicken stock
1/2 cup whole milk
1/2 cup half and half
4 1/2 cups rostisserie chicken, skin discarded, shredded
1 cup frozen peas, thawed
1 tablespoon poppy seeds
1 cup sour cream
1 1/2 cups butter crackers, crushed
1/2 cup pecans, roughly chopped
Kosher salt and freshly ground black pepper to taste

Preheat the oven to 350ºF. Grease a 9x13-inch baking dish with 1 tablespoon of butter.
Heat a Dutch oven over medium-high heat and add olive oil and butter.
Add onion, carrots and celery and cook until almost translucent, about 4 minutes.
Add cayenne, thyme and season with salt and pepper. Cook an additional minute.
Sprinkle over the flour and stir to coat the vegetables.
Add the chicken stock and bring to a simmer.
Add the whole milk and half and half and simmer until thickened, about 3 minutes.
Stir in the chicken, peas, and poppy seeds. Season with salt and pepper again and allow to cool.
Once cooled, add the sour cream and stir to combine. Pour the mixture into the baking dish.
Combine the crackers and pecans in a small bowl.
Sprinkle over the top of the casserole.
Bake for 20-25 minutes or until the crackers are golden brown and the filling is bubbling.
Allow to cool slightly and serve.
Tip: Make the chicken filling mixture and topping in advance but store separately.
Assemble the casserole right before baking with the topping, bake and serve.