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, half the pineapple, 1/2 tsp. salt and 1/4 tsp. pepper.

Cook, covered, until meat is tender and shreds easily, 7 to 8 hours on low or 4 to 5 hours on high.

When the beef has 25 minutes left to cook, thinly slice the remaining onion. In a medium bowl, toss the onion, jalapeno, lime juice, remaining pineapple and 1/4 tsp. each salt and pepper; fold in the avocado and cilantro.

Using two forks, shred the meat, then stir it into the cooking liquid. Fill the tortillas with the beef mixture and top with the pineapple salsa.