Preheat oven to 400°. In a large saucepan, heat butter over medium heat. Add mushrooms, carrots and onion; cook and stir until tender., Stir in flour until blended; gradually stir in broth and milk. Bring to a boil, stirring constantly; cook and stir 2 minutes or until thickened. Stir in chicken, peas, pimientos and salt; heat through. Transfer to a greased 11x7-in. baking dish., For topping, in a large bowl, whisk flour, baking powder, sugar, salt and cream of tartar. Cut in butter until mixture resembles coarse crumbs. Add milk; stir just until moistened., Turn onto a lightly floured surface; knead gently 8-10 times. Pat or roll dough to 1/2-in. thickness; cut with a floured 2-1/2-in. biscuit cutter. Place over chicken mixture. Bake, uncovered, 15-20 minutes or until biscuits are golden brown.