Heat a gas grill to medium high heat.

Combine the Vulcan's Fire Salt, Back of the Yards rub, cumin, and garlic powder in a small bowl. Mix well and rub all over the flank steak.

Place the steak on the grill with the thermometer pressed into the center of it if using.

Drizzle the bell peppers and onion quarters with olive oil and sprinkle with salt and pepper.

Turn the vegetables every 1-2 minutes or until the onions have grill marks on all sides and the peppers have char marks and the skin is beginning to lighten. Remove to a cutting board and cut into thin strips.

Cook the flank steak until the thermometer reads 135. Remove from the grill and allow to rest for 5 minutes. Slice into thin strips.

Serve the vegetables and steak with tortillas, cheese, sour cream, salsa, and guacamole.