In a large mixing bowl, stir together the shredded chicken with the diced pineapple, chopped celery, sliced almonds, and fresh cilantro. If you enjoy onions' flavor, toss in your finely diced red onions now.
In a small bowl, whisk together the Greek yogurt with lemon juice, dijon mustard, and kosher salt until smooth.
Pour the dressing over your chicken mixture and gently stir to combine all ingredients thoroughly.
To serve, lay out your butter lettuce leaves on each croissant half. Spread one-quarter of your chicken salad mixture over each lettuce layer before topping with the other half of the croissant.