Puree the grapes in a blender then strain through a sieve, pressing on the solids to extract as much juice as possible. You should have about 2 1/2 cups of juice.
Preheat the oven to 400 degree Fahrenheit. Season the chicken on all sides with salt, pepper and fennel spice.
Heat a large ovenproof skillet over moderately high heat. When the skillet is hot, add the olive oil.
Add the chicken breasts, skin side down, and brown well on all sides, about seven minutes total.
Transfer the skillet to the oven and cook until the chicken is done throughout, 12 to 15 minutes.
Transfer the chicken to a serving platter and pour off all but 1 tablespoon of the fat in the skillet.
Add the shallot to the skillet and return to moderate heat. Cook until softened, then add the rosemary and cook briefly to release its fragrance.
Add 2 cups of the grape juice and simmer briskly until reduced by half.
Add the stock and any collected juices from the chicken platter and simmer until the mixture has reduced to a creamy, sauce-like consistency. The total volume of the sauce will be a little more than 1 cup.
Cut the chicken breasts in half with a heavy knife or cleaver and return them to the platter. Spoon the sauce over and around them.
Serve immediately.
Put the fennel seed, coriander seed and peppercorns in a small, heavy skillet over moderate heat. Watching carefully, toss the seeds frequently so they toast evenly. When they are light brown and fragrant, pour them onto a plate to cool. (They must be cool before grinding, or they will gum up the blender blades.)
Pour the cooled seeds into a blender and add the salt. Blend to a powder, removing the blender from its stand and shaking it occasionally to redistribute the seeds. Store in an airtight container away from light and heat for up to 4 months. or freeze for up to 1 year.
