Combine the mustard, honey and tarragon in a small bowl; whisk to blend.
To assemble, spread a layer of the honey mustard on the inside of the ciabatta. Layer spinach leaves on the bottom piece of bread. Place 2 to 3 slices of turkey (about 3 oz.) on the sandwich. Top with slices of brie and granny smith apple. Sandwich together and serve with additional honey mustard tarragon sauce for dipping.
Notes
I like these sandwiches served cold or pressed. To press, preheat a nonstick Panini press. Grill the sandwiches for about 5 minutes, until the bread is toasted and the cheese starts to melt. Remove from press and serve.