In 12-inch nonstick skillet, heat oil over medium-high heat. Cook onion in oil, stirring occasionally, until brown. Add mushrooms. Cook 6 to 8 minutes or until tender. Reduce heat to low. Add taco seasoning mix and water. Cook about 5 minutes or until water is absorbed. Remove from heat.
Spoon filling on tortillas. Add toppings. Roll up tortillas.