In a large shallow bowl, whisk together the tamari, olive oil, lime juice, cilantro, cumin, chipotle powder and garlic powder. Add the sliced mushrooms and stir until evenly coated. Allow to marinate at room temperature for 10 minutes, then add the sliced onion and peppers to the bowl and stir everything together.
Warm a large skillet over medium heat. Add the marinated vegetables to the skillet and cook for about 7 minutes, until tender. (Depending on the size of your skillet, you may have to cook them in two separate batches.) Remove from heat.
Serve the cooked vegetables warm with tortillas, guacamole and other desired toppings (such as salsa, Greek yogurt, shredded cheese). Leftovers can be stored in an airtight container in the refrigerator for up to 2 days.
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