Put the steak in a baking dish and toss with the pickled jalapeno mixture; let marinate at room temperature, 1 hour.
Preheat a grill or grill pan to medium high. Brush the jalapeno mixture off the steak; season the steak with salt and pepper and grill until charred, 3 to 4 minutes per side. Transfer to a cutting board and let rest.
Secure each onion slice horizontally with a wooden skewer. Brush the onion slices with olive oil and season with salt and pepper. Grill, turning occasionally, until soft, 10 to 15 minutes. Meanwhile, put the poblanos and fresh jalapenos on the grill and char on all sides, about 8 minutes. Set aside until cool enough to handle, then peel off the skins, stem and seed. Cut the poblanos and jalapenos into strips. Once the onion slices are cool enough to handle, remove the skewers and separate into individual rings.
Preheat a large skillet over medium heat. Thinly slice the steak on the bias, against the grain. When the skillet is hot, coat the bottom with 2 tablespoons olive oil. Add the garlic and oregano and saute about 1 minute. Add the steak, onion and pepper strips and toss to heat through. Season with salt and pepper. Serve with tortillas, guacamole and pico de gallo.
Photograph by Kat Teutsch
Recipe courtesy Aaron Sanchez for Food Network Magazine