1In small bowl, mix all slaw ingredients; set aside.
2Heat taco shells as directed on box.
3Meanwhile, in large skillet, heat oil over medium-high heat. Cook mushrooms in oil 3 to 5 minutes, stirring occasionally, until they begin to brown. Add onion; cook 2 to 3 minutes longer. Stir in chili powder and grill seasoning.
4To serve, spoon mushroom mixture into warmed taco shells; top with avocado slices and spicy cabbage slaw.