Combine the marinade ingredients in a large bowl or container. Season with S&P as desired. Whisk to combine well. Place the skirt steak in the marinade, tossing to coat the steak well. Set aside as you preheat the grill or seal/cover & marinate in the refrigerator for up to 2 days.
Preheat the grill for high direct heat grilling, 550-600 degrees F. Once preheated, clean the cooking surface by firmly brushing a wire brush over the grates. Meanwhile, place the bell peppers & onions in a mixing bowl or on a small baking sheet. Drizzle with the remaining 1 tablespoon avocado oil. Season with S&P as desired. Toss to coat. Set aside for grilling.
Place the steak on the grill grates, draining & discarding any excess marinade. Arrange the vegetables around the steak. Close the grill lid. Grill the steak 2-4 minutes per side for preferred doneness – about 3 minutes per side yields a nice medium-rare. Grill vegetables 3-4 minutes per side, or until nicely charred. Transfer to a plate or small baking sheet. Let the steak rest for at least 5 minutes before carving.
Once cool enough to handle, slice the grilled skirt steak & fajitas veggies into strips, slicing steak against the grain. Serve immediately as desired.