
Preheat the oven to 400°F, and grease an 11x7-inch baking dish. Melt the butter in a large saucepan over medium heat. Add the mushrooms, carrots and onions. Saute and stir the veggies until they're softened.
Sprinkle flour over the butter and veggie mixture, and stir over medium heat until it's completely blended in. Pour in the chicken broth and milk. Bring to a boil, stirring constantly. Cook and stir the mixture for two minutes until it's thickened.
Add in the chicken, frozen peas and pimientos; continue stirring until all of the ingredients are heated through. Pour the mixture into the prepared baking dish.
Whisk together the flour, baking powder, sugar, salt and cream of tartar in a large mixing bowl. Use a pastry cutter (a must-have baking tool) or two forks to cut in the cold butter until the mixture looks like coarse crumbs. Stir in just enough milk to moisten the mixture. Turn the mixture out onto a lightly floured surface; knead the dough 8 to 10 times to bring it together. Pat it out into a slab about 1/2-inch thick. Use a 2-1/2-inch cutter to cut out the biscuits. Editor's Tip: If you don't have a pastry cutter, no problem! Simply cut the slab of dough into equal squares or rectangles.
Arrange the biscuits over the top of the chicken mixture in the baking dish. Bake the casserole for 15 to 20 minutes, until the biscuits look puffed and golden brown.