In a small bowl, whisk together the lime juice, minced garlic, ground cumin, chili powder, sea salt, black pepper, and half of the olive oil to create a marinade.
Place the flank steak in a shallow dish or resealable bag and coat thoroughly with the marinade. Let it rest for at least 20 minutes at room temperature.
While the steak marinates, place the whole poblano pepper under a broiler or over an open flame, turning occasionally until the skin is charred and blistered on all sides.
Place the charred pepper in a bowl and cover with plastic wrap for 5 minutes to steam. Peel off the skin, remove the seeds, and slice the pepper into thin strips.
Heat the remaining olive oil in a cast-iron skillet over medium-high heat. Add the sliced red onions and sauté until they begin to soften and caramelize.
Move the onions to the side of the skillet and add the flank steak. Sear the steak for 4-5 minutes per side for medium-rare, or until it reaches your desired level of doneness.
Remove the steak from the skillet and let it rest on a cutting board for 5 minutes before slicing thinly against the grain.
Toss the roasted poblano strips with the caramelized onions in the skillet to warm through.
Serve the sliced carne asada topped with the poblano and onion mixture, garnished with fresh chopped cilantro.