Preheat the oven to 350 degrees F. Butter a large casserole dish.
In a large saucepan over medium-low heat, add milk, thyme, rosemary, sage, and garlic. Bring to a simmer and cook for 15 minutes (do not let the mixture come to a boil). Strain.
While the milk is simmering, heat 2 tablespoons of olive oil in a large pot over medium-high heat. Season your chicken with salt and pepper. Add the chicken to the pot and brown on each side for 2-3 minutes until golden brown. Transfer the chicken to a sheet pan and finish cooking in the oven, about 25 minutes. When the chicken is finished, transfer to a cutting board and allow to cool before shredding. Turn the oven temperature up to 400 degrees.
Turn the heat down to medium-low and and add 2 tablespoons of olive oil to the chicken drippings. Add celery, carrots, onion, potato, and bay leaves. Sprinkle with salt and pepper. Cook the vegetables, stirring occasionally, until tender, about 30 minutes.
Remove the bay leaves and add the butter. Once the butter has melted, add the flour to create a roux; cook for 1-2 minutes.
Stir in the strained milk, stirring until smooth. Add chicken stock, Bon Appetit seasoning, worcestershire sauce, and a pinch of salt and pepper. Allow the mixture to come to a gentle simmer and thicken. If the mixture gets too thick, stir in some more chicken stock.
Once the mixture thickens, add the frozen peas, corn, shredded chicken and chopped thyme. Taste the mixture to see if it needs more salt and pepper.
In a large mixing bowl, whisk together the flour, baking powder, salt, and sugar.
Cut the butter into the dry ingredients, either using a fork or a pastry cutter, until the mixture resembles coarse crumbs. Add the buttermilk and egg and stir until just combined.
Transfer the mixture to a lightly floured surface and gently knead until the dough comes together, making sure not to over work the dough.
Roll out the dough to about a ½-inch thick. Cut out biscuits with a round biscuit cutter.
Place the biscuits directly on top of the chicken mixture, spacing them 1-inch apart. Place the dish in the oven and bake 20-25 minutes, or until the biscuits are golden brown and the casserole is bubbly.
Recipe by The Messy Baker Blog at http://themessybakerblog.com/2013/01/20/chicken-biscuit-pot-pie-casserole-sundaysupper/