Add all marinade ingredients into a casserole dish and incorporate well. Add all skirt steak pieces and be sure that all pieces are covered in marinade. Marinate for 7+ hours in the refrigerator covered (if you're short on time, I'm sure you can go for 30-60 minutes). 24 hours is optimum time.
Remove from refrigerator at least 20 minutes before you're going to grill the steak. Remove from casserole dish and allow to sit on a plate.
Heat grill pan on low for 5-8 minutes. Then, turn heat up to medium. Pat pieces of skirt steak dry with paper towels and place onto grill pan. You may need to do this in batches in order not to crowd the beef in the grill pan. Cook 4-8 minutes on each side depending on thickness.
Allow beef to rest on a clean plate with tented foil for at least 5 minutes before slicing the beef against the grain.
While the beef is resting, add onions and peppers to the grill pan and allow to cook until onions and peppers are soft, be sure to mix with tongs or wooden spoon frequently.