For the marinade: Add all the marinade ingredients into a bowl, stir to combine.
For the fajitas: Place the steak in a large resealable plastic bag and pour the marinade over. Press out as much air as possible and seal the bag. Shake the bag to distribute the marinade then lay the bag flat and refrigerate for 2-3 hours (flipping it halfway through).
20-30 minutes before cooking, remove steak from the fridge. Remove steak from the marinade and shake off excess liquid (or lightly pat dry with a paper towel).
Preheat Ooni Koda 16 oven on high for 15-20 minutes.
Place the Ooni Dual Sided Grizzler Plate (flat side up) in the middle of the of oven until cast iron temperature reaches 600°F (4-6 minutes).
Carefully remove Grizzler Plate from the oven (using handles) and place on wooden base. Place the steaks closer to right hand side of the plate and return to oven as close to the right-hand side as possible away from the flames. Cook the steak for 2 minutes, then remove the Grizzler plate from oven, rotate the steaks (front to back) and exchange positions (left to right). Return to oven and cook for 2 minutes more. Flip steaks and cook for 1.5 minutes, then rotate and exchange positions (check internal temperature using an instant read thermometer). Return steaks to oven and cook for about 1.5 minutes more until internal temperature reaches between 125-130°F. Remove steaks to a platter and tent with foil while you cook the vegetables (steak temperature will rise to 135°F - medium rare as it rests).
Pour 2 tablespoons of oil onto the Grizzler plate, add onions, season with salt and pepper and mix to combine. Adjust the temperature control on the Koda 16 to the lowest setting and return Grizzler pan with onions in oven on the right-hand side.
Cook onions for about 6 minutes (stirring occasionally) until softened. Add the peppers & jalapeños, season with salt & pepper, add a splash of olive oil and mix carefully to combine with onions (the Grizzler plate will be full, but since the veggies are cooking from the top and bottom, they will cook evenly). Return the Grizzler plate to the oven (keeping to the right) and adjust temperature back to high. Let them cook undisturbed for 2 minutes. Stir then continue cooking, stirring occasionally, until they are softened and have some browned spots, 4 to 6 minutes.
While the peppers finish cooking, begin thinly slicing the steaks across the grain.
When peppers are done, remove the Grizzler plate to the wooden base and place sliced steak on top of steaming hot peppers & onions.
For serving: Surround the hot Grizzler plate (on its base) with all the accompaniments in separate dishes so that people can assemble their own fajitas.