Preheat oven to 450 degrees F. Coat a 2-quart square or rectangular baking dish with cooking spray.
In a large skillet melt 2 tablespoons butter over medium heat. Add the next three ingredients (through onion); cook 8 minutes or until onion is tender, stirring occasionally. Add chicken; cook 5 minutes or just until browned, stirring occasionally. Stir in the next four ingredients (through pepper); gradually stir in 1 1/2 cups milk. Cook and stir until thickened and bubbly. Stir in peas; cook and stir 1 minute more. Cover and keep warm.
For biscuit topper, in a medium bowl stir together the next three ingredients (through 1/4 teaspoon salt). Using a pastry blender, cut in 1/4 cup butter until mixture resembles coarse crumbs. Make a well in center of flour mixture. Add 1/2 cup milk all at once; stir just until moistened.
Transfer chicken mixture to prepared baking dish. Drop biscuit topper in nine mounds on top of chicken mixture. Bake 20 minutes or until filling is bubbly and biscuits are golden. Let stand 10 minutes before serving.