Place first 8 ingredients in a large plastic bag.
Place the skirt steak in the bag, press out the excess air and seal. Lightly massage the bag to make sure the marinade coats the meat and then refrigerate for at least one hour or overnight.
Heat a large cast-iron skillet over medium-high heat. Add ghee and then place the steak in the pan. Sear 5 minutes each side and remove from the pan. Set aside and cover with foil to rest.
Meanwhile, add the sliced bell peppers, onion and jalapeños to the skillet. Add the reserved marinade that was used for the meat and sauté vegetables for 4 to 5 minutes. Thinly slice the skirt steak across the grain and place on a large serving plate.
Top with the cooked fajita vegetables and avocado slices. Serve over fresh spinach.