1 large red onion 1 1/2 lb flank steak, cut crosswise into 4 pieces 1 Tbsp chili powder 1 tsp ancho chile powder (optional) 1/2 pineapple (about 2 lb), peeled, cored, and cut into 1/2 in pieces Kosher salt and pepper 1 jalapeņo (seeded for less heat), thinly sliced 2 Tbsp fresh lime juice 1 avocado, diced 1/2 cup fresh cilantro 8 small flour tortillas, warmed

1. Chop 3/4 of the onion and refrigerate the rest until ready to use. In a 5 to 6 qt slow cooker, toss the chopped onion, beef, chili powder, ancho chili powder, half the pineapple, 1/2 tsp salt and 1/4 tsp pepper.
2. Cook, uncovered, until the meat is tender and shreds easily, 7 to 8 hours on low or 4 to 5 hours on high.
3. 25 minutes before serving, thinly slice the remaining onion. In a medium bowl, toss the onion, jalapeņo, lime juice, remaining pineapple and 1/4 tsp each salt and pepper. Fold in the avocado and cilantro.
4. Using two forks, shred the meat, then stir it into the cooking liquid. Fill the tortillas with the beef mixture and top with pineapple salsa.