In a large bowl, stir together mayonnaise, tarragon, mustards, lemon juice, salt, and pepper. Add chicken, stirring until well combined.
Using a serrated bread knife in a sawing motion, cut croissants in half horizontally. Place arugula on bottom half of croissants. Top with chicken salad, and cover with top half of croissants.
Notes
*We used Grey Poupon Dijon Mustard and Maille Old Style Whole-Grain Dijon Mustard.
MAKE-AHEAD TIP: Chicken salad can be made a day in advance, placed in an airtight container, and refrigerated until ready to use.
Recipe by TeaTime Magazine at https://teatimemagazine.com/tarragon-chicken-salad-sandwiches/