In a small bowl, stir together the marinade ingredients. Coat the mushrooms with the marinade (I just used my hands), and let them sit for 30 minutes.
Meanwhile, prep the other ingredients. In a small bowl, mash the guacamole ingredients together using a fork. Set aside until needed.
Preheat your BBQ/grill to high. Reduce the heat to medium-high and grill the mushrooms for about 6 minutes/side, turning once, for a total of 12 minutes (or until they're cooked through). Just prior to serving the tacos, warm the tortillas on the grill for about 10-20 seconds/side or until they've got char marks to your liking.
Once the mushrooms are cooked, slice them into smaller pieces. Top the tacos with all the toppings and the mushrooms. Eat immediately.