Combine all marinade ingredients in a large shallow dish or zip-top bag, tasting and adjusting seasonings as needed. Thoroughly coat the steak in the marinade, cover, and refrigerate for 2–4 hours or ideally overnight; remove from the fridge 30 minutes before cooking. Heat a large cast iron skillet or grill pan over high heat, allowing it to preheat for a couple of minutes. Place the steak in the pan, searing for 3–5 minutes per side depending on your preferred doneness; medium-rare is achieved around 3 minutes per side. Lower heat slightly if the pan becomes too hot. Transfer the cooked steak to a plate and tent with foil, letting it rest. Add the remaining tablespoon of oil to the same pan over high heat, then sauté the onions, poblano, and bell peppers, tossing occasionally and cooking for about 2–3 minutes until tender-crisp; season with a touch of salt and pepper. If necessary, cook vegetables in batches to prevent steaming. After resting, slice the steak thinly against the grain, angling the knife for optimal tenderness, and optionally return the slices to the pan with the veggies. Serve the fajitas in warm tortillas or over rice bowls, garnished with your choice of toppings.