Step 1

In a large bowl, stir together the chicken, celery, scallions, lemon zest and juice, mayonnaise, parsley, and mustard until combined. Taste, and season with salt, pepper and more lemon zest and juice, as desired. You should have about 2 1/2 cups.

Step 2

To serve, place a lettuce leaf on the bottom half of each croissant, spoon on some chicken salad, add tomato slices, sprinkle lightly with salt and pepper, and top with the other half of the croissant. Serve right away.