Preheat the oven to 375°F. Spray a shallow roasting dish with cooking spray and arrange thickly sliced onions and half of the fresh herbs inside.
Place the chicken breasts on top, squeeze fresh lemon juice over them, and season with salt and pepper.
Roast uncovered for 35 minutes until the chicken is cooked through. Let the chicken cool and chop it for the salad, discarding the herbs and onions.
In a large bowl, mix the chopped chicken with celery, shallot, cranberries, toasted pecans, fresh tarragon leaves, sour cream, mayonnaise, and curry powder.
Stir well, then taste and adjust the seasoning with salt and pepper as needed.
Cover the bowl and chill the salad for several hours or overnight to allow flavors to meld.
For tea sandwiches, slice bread into small finger portions.
Optionally, spread a small amount of mayonnaise on the bread, then place a tablespoon of chicken salad onto each piece.