In a large bowl/baking dish, whisk together all marinade ingredients until combined.
Place skirt steak in marinade dish, flipping over a couple times until well-coated in marinade mixture. Cover and place in fridge to marinate for 4 hours to overnight.
When skirt steak has finished marinating and ready to cook, remove from marinade (discard marinade) and pat steak dry with a paper towel.
Next, heat grill to high (~600℉—700℉).
Once heated, place skirt steak onto grill. Cover with grill lid and let steak sear/cook for ~5 minutes on each side until reaching internal temperature of ~170℉ (for medium cooked), or adjust time until cooked to desired level.
Remove steak from grill (but don’t turn grill off) and let skirt steak rest on a plate or cutting board for ~5–10 minutes, prior to cutting so as to retain moisture and flavor.
While the meat rests, place a large cast-iron skillet on still heated grill — once skillet has heated up, add oil and veggies. Cook for ~10 minutes, or until tender, but firm, and lightly charred.
While the veggies are cooking, also place tortillas straight onto the grill and lightly char/heat them on each side (~1-2 minutes per side). Then, set tortillas aside.
Once rested, slice steak against the grain into 1/4” thick strips (drizzle any juices on top of cut strips for added flavor and moisture.
When ready to eat, assemble fajitas by placing veggies and steak strips onto tortillas, top with cilantro, squeeze of lime, and any optional ingredients.
Serve, store, and enjoy!