Combine the marinade ingredients in a small mason jar, seal, and shake vigorously until combined. Alternatively, add to a small bowl and whisk until combined.
Add the flank steak to a large plastic bag or glass container. Pour the marinade over the steak and seal/cover. Marinate for 1-2 hours.
While the steak is marinating, make the salsa by combining all ingredients in a small bowl. Stir together, cover, and refrigerate until ready to use.
Place flank steak on a hot grill and cook for 4-5 minutes per side, or until it reaches desired doneness.
Remove from the grill and let the steak rest for at least 10 minutes, then slice against the grain into strips.
Top sliced flank steak with the pineapple salsa and serve.