1. In a large glass dish, combine pineapple juice and soy sauce. Add steak, turning to coat, and marinate at room temperature, 20 minutes.

2. Meanwhile, heat broiler, with rack in top position. Place poblanos and onion on a rimmed baking sheet and broil until charred, 7 to 10 minutes, flipping halfway through. When poblanos are cool enough to handle, rub with a paper towel to remove skins. Remove stems and seeds and discard. Roughly chop poblanos and onion and place in a medium bowl. Stir in oil, lime juice, and cilantro; season relish with salt and pepper.

3. Heat a grill or grill pan to high. Clean and lightly oil hot grill. Remove steak from marinade, letting excess drip off. Grill steak 3 to 5 minutes per side for medium-rare, flipping once. Transfer steak to a cutting board and let rest 5 minutes before thinly slicing against the grain. Serve steak with relish.