In a small bowl, combine the 1 tablespoon salt, fennel seeds, 1 teaspoon pepper, lemon zest, and 1 tablespoon of the olive oil. Rub this mixture generously over the chicken. Place the chicken, skin-side up, on a rimmed baking sheet and let it stand for at least 1 hour.
About 15 minutes before you are ready to cook the chicken, heat the oven to 475°F.
Transfer the chicken to the oven and roast it for 20 minutes.
In a small bowl, toss the grapes with the remaining 2 teaspoons olive oil, the sugar, and salt and pepper to taste. Scatter the grapes around the chicken, and roast until the chicken is just cooked through and the grapes are lightly caramelized, 20 to 25 minutes.
Transfer the chicken to a cutting board to rest. Spoon the grapes into a bowl. Place the baking sheet over two burners on medium-high heat. Add the vinegar to the pan juices and scrape up the browned bits from the bottom of the baking sheet. Pour the mixture into a small saucepan and warm it over medium heat. Whisk in the butter.
Carve the chicken and top it with the grapes and spoonfuls of the sauce.
Total time: 1-3/4 Hours + at least 1 hour marinating