In small bowl, mix chili powder, garlic powder, onion powder, cumin and salt. Stir in water and set aside.
In large skillet over medium-high heat, heat oil. Add chopped mushrooms and ground beef and cook thoroughly, about 7-10 minutes. Carefully drain excess grease, if necessary.
Add seasoning mixture to mushroom-meat mixture and stir to combine. Bring to simmer and cook 3-5 minutes.
To assemble, add about 1/4 cup mushroom-meat mixture to bottom of each taco shell. Layer shells with cheese, tomatoes and lettuce, if desired.