Marinate the flank steak in the pickled jalapenos, at room temperature, for 1 hour.
Preheat a grill or grill pan to medium-high.
Season the steak with salt and pepper, to taste, and grill until nicely charred, about 3 to 4 minutes on each side. Remove from the grill and allow the steak to rest on a cutting board while preparing the rajas.
Secure each piece sliced onion horizontally with wooden skewers. Brush both sides with olive oil and season with salt and pepper, to taste. Grill the onions, turning occasionally, until soft and charred, about 10 to 15 minutes. Put the poblano and jalapeno chiles on the grill and char them on all sides. Once the chiles are blackened on all sides, set aside to cool. When cool enough to handle, remove the charred skin, stem and seeds. Cut the chiles into strips. Once the onions are cool enough to handle, remove the skewers and separate the onions into rings.
Preheat a large cast iron pan or a large heavy-bottomed skillet over medium heat. Slice the flank steak, on the bias, into thin strips. When the pan is hot, coat the bottom of the pan with some olive oil. Add the sliced garlic and dried oregano, and saute until fragrant, about 1 minute. Add the sliced steak, onions and chile strips to the pan to heat through. Transfer to a serving platter and serve piping hot with flour tortillas, guacamole and pico de gallo.