To Marinate The Steak:

1. Combine all marinade ingredients in a large ziplock bag or in a baking dish. Add the flank steak, and toss with the marinade until it is completely coated. Seal (or cover with plastic wrap or foil, if using the dish) and refrigerate for 15 minutes or up to 1 day.

To Make The Tacos:

1. When you’re ready to make the tacos, remove the steak from the refrigerator and let it rest for at least 20 minutes to come to room temperature.
2. Heat your saute pan (or grill pan) over medium-high heat. Remove the flank steak from the marinade (discard the marinade) and add the steak to the pan. Cook on each side for 4-5 minutes, turning once with tongs, until it reaches your desired level of doneness. Transfer the steak to a separate plate, and let it rest for at least 10 minutes. Then slice it into thin strips on the diagonal.
3. While your steak is resting, add the oil to the saute pan and reduce heat to medium-high. Add the onions and poblano peppers, and saute for 5 minutes until the onions are soft. Add the mushrooms and continue sauteing until they are cooked, about 2 minutes. Add the garlic, salt, cumin and black pepper and saute 1-2 more minutes until the garlic is fragrant. Season with additional salt and pepper, if needed. Pour the veggie mixture onto a serving platter and set aside.
4. Return pan to heat and add the corn tortillas, warming for about 10 seconds per side to soften. Remove pan from heat.
5. Assemble tacos by layering the sliced steak, veggies, cilantro on top of the warmed tortillas. Serve with avocado, lime wedges, salsa verde and optional additional toppings.