1Heat oven to 375°F. Spray 8-inch square (2-quart) glass baking dish with cooking spray. In 8-inch skillet, cook beef and onion over medium-high heat 5 to 7 minutes, stirring occasionally, until thoroughly cooked; drain. Stir in taco seasoning mix until blended.
2In medium bowl, stir Bisquick mix and water until soft dough forms. Using fingers dipped in Bisquick mix, pat dough in bottom and 1/2 inch up sides of baking dish.
3In same bowl, mix sour cream, mayonnaise and cheese until blended. Layer beef mixture, tomato and chiles over dough; spoon sour cream mixture over top.
4Bake 25 to 30 minutes or until edges of dough are light golden brown.