Season the chicken with gray salt and fennel spice, pressing the spice into the chicken breasts.
In a deep-sided oven-proof skillet, heat the olive oil. Add the chicken, skin side down, and saute for 3 minutes to brown. Place the pan in the oven, uncovered, for 10 minutes.
Remove from the oven and turn the chicken over and place skin side up on a baking sheet. Return chicken to the oven until cooked through, about 15 to 20 minutes. Remove the pieces to a platter.
Using the same pan used for the chicken, add the shallot to the fat in the pan. Add salt and pepper, cook for 2 minutes. Add the chopped rosemary and let sweat for 1 minute, then add the grape juice. Reduce by half and add the chicken stock. Reduce the sauce by half again, for about 1 cup of sauce. Add the grapes and cook just to heat through, about 1 minute.
Spoon the grapes and sauce over the chicken and garnish with sprigs of fresh rosemary.
Fennel Spice Rub:
Put the fennel seeds, coriander seeds, and peppercorns in a heavy pan over medium heat. Watch carfelly, tossing frequently so the seeds toast evenly. When light brown and fragrant, pour the seeds onto a plate to cool. They must be cool before grinding, or they will gum up the blades.
Pour the seeds into a blender or spice grinder and add the salt. Blend to a fine powder, shaking the blender/grinder occasionally to redistribute the seeds. Store in a tightly sealed glass jar in a cool, dry place, or freeze.