Melt butter in a saucepan. Add onion; cook until tender. Stir in next six ingredients; mix well. Cook until thickened and bubbly. Stir in chicken, vegetables and pimento; heat until bubbly. Divide mixture into six individual casseroles. Cut each biscuit into quarters; place four quarters on top of each casserole. Place on shallow baking pan; bake at 450°F/230°C for 8 to 10 minutes or until biscuits are lightly browned.