If you don't have pre-cooked chicken, poach or roast 1 large chicken breast (about 10–12 oz) until internal temperature reaches 165°F (74°C). Let rest, then shred or finely chop into bite-sized pieces to measure ~3 cups. Cool before mixing.

In a medium mixing bowl combine mayonnaise (3 tbs) and Greek yogurt (2 tbs). Whisk in Dijon mustard (1 tsp), lemon juice (1 tbs), celery seed (0.5 tsp if using), and sugar (0.25 tsp).

Add the shredded chicken, diced celery (1 cup), finely diced red onion (0.25 cup), and chopped parsley (2 tbs) to the dressing. Fold gently to coat all pieces evenly without mashing the chicken.

Season the mixture with kosher salt (0.75 tsp) and freshly ground black pepper (0.5 tsp). Taste and adjust — add more lemon for brightness, more mayo for creaminess, or a pinch more salt if needed. Chill in refrigerator for at least 20 minutes to let flavors meld (or up to 2 days).

Sandwich Build

Prepare tomatoes: slice tomato very thinly. Lightly season both sides of slices with a pinch of salt (0.25 tsp divided) and a grind of black pepper (0.125 tsp). Let sit briefly while you prepare croissants.

If toasting croissants: heat a skillet over medium-low. Spread about 0.5 tsp butter on each cut side of the croissant. Place croissant halves butter-side-down in the skillet and toast until golden and slightly crisp, 1–2 minutes. Alternatively, warm in a 350°F (175°C) oven for 5–7 minutes.

Assemble sandwiches: place 1–2 leaves of lettuce on the bottom half of each croissant. Spoon about 3/4 cup of the chilled chicken salad onto lettuce (adjust to preference). Top chicken salad with 3–4 tomato slices, then place the croissant top on.

Serve immediately while croissants are warm and crisp, or wrap chilled for transport. Each sandwich can be cut in half for easier handling. Store leftover chicken salad in an airtight container in the refrigerator for up to 2 days.