• In a large skillet, melt the butter with the olive oil over medium heat.
• When it is hot, add the onions, carrots, and celery. Saute for about 7-10 minutes, until the carrots are tender. Sprinkle with a little salt and pepper.
• Now we're going to make a roux. At this point you can either remove the vegetables from the pan with a slotted spoon (reserving in a bowl) or you could try my work-around. Tilt the pan to the side so that all the butter and fat is at the bottom. Push the veggies up (see photos).
• Add the flour to the butter in the pan. Whisk together and cook for about 1 minute to cook out the flour taste.
•  Pour the chicken broth VERY slowly into the flour, keeping the pan tilted. Pour a little bit, then whisk so that the liquid is absorbed, then stir some more. Take your time, you don't want lumpy sauce. 
• Once you have added all the broth and have a nice white sauce, you can stir the vegetables together it. 
• Slowly stir in the milk. 
• Season with 1 and 1/2 teaspoons salt, 1 teaspoon pepper, and 1/2 teaspoon celery seed.
• Cook, stirring occasionally for 3-4 minutes, or until it has reached a thicker consistency.
• Stir in the chicken and frozen peas. Turn off the heat. 
• Pour the mixture into a 9x13 inch casserole dish.
• Preheat the oven to 400 degrees F.

Make the dough for these homemade Buttermilk Biscuits, skipping all the rolling steps. Once you have kneaded the dough together, proceed here:

• Separate the dough into 12 pieces and flatten them a little bit with your hands into a rough disc. It doesn't need to be perfect, and don't go too crazy, you want them kind of thick. (But not dome-y.) See photos. 
• Place each biscuit on top of the chicken mixture in the 9x13 pan. 
• Brush each biscuit with buttermilk or milk.
• Bake at 400 for 25-30 minutes. The tops of the biscuits should be golden brown and the casserole should be bubbly.