Roast the chicken
Preheat the oven to 350° F.
Place the chicken breasts on a sheet pan and rub the skin with olive oil. Sprinkle with salt and pepper.
Roast for 35 to 40 minutes, until the chicken is just cooked. Set aside until cool enough to handle.
Remove the meat from the bones, discard the skin, and dice the chicken into bite-size pieces
For the dressing, whisk together the mayo, wine, both mustards, parsley, dill, and a strong pinch of sea salt and a grind or two of pepper
Add enough dressing to the warm chicken to moisten well. Taste and adjust seasoning if needed.
Add the capers and grapes, mixing gently to combine. (It’s best if you’re able to refrigerate the chicken salad for a few hours in order for the flavors to blend)
Store in an airtight glass container in the fridge, use within three days
Build your sandwich
Divide the chicken salad evenly between the four slices of bread and spread to cover.
Layer the cucumber, tomato, and greens on two of the slices, and top with the remaining two.
Slice in half and enjoy right away.