In a shallow bowl, combine the garlic, fajita seasoning, grape-seed oil and cilantro. Toss the steak to coat in the marinade. Cover and marinate for 30 minutes.

Meanwhile, heat a 12-inch cast-iron skillet over medium-high heat. Place the butter, olive oil, and onion in the skillet. Season with salt, stir and cook until the edges of the onion are deeply golden yet still a bit firm, about 10 minutes. Add the peppers and cook for 5 - 6 minutes more, depending on how you want them. Transfer the onion and peppers to a small dish.

Working in two batches, sear the steak on both sides for 3- 4 minutes on each side. Transfer to a cutting board and let rest for 5 to 8 minutes before slicing into thin strips. Return the strips and any juices from the cutting board to the skillet, squeeze in the juice from the lime wedges and cook for 3 to 4 minutes more. Remove from the heat and tent with aluminum foil to keep warm. Serve with warm tortillas, guacamole, cotija cheese and pico de gallo.