Preheat the oven to 400°F. Pat the chicken thighs dry and season with kosher salt and pepper. Heat a braiser or other oven safe skillet over medium high heat. Add olive oil to the braiser. Place the chicken thighs skin side down in the braiser. Sear the chicken until the skin is crispy and golden brown.
Flip the chicken thighs over and sear on the other side until golden brown. Remove the chicken to a plate and set aside. Remove all but a tablespoon of fat from the braiser.
Lower the temperature to medium and add the shallots. Sauté for 2 to 3 minutes until the shallots are softened. Gather the garlic, red wine vinegar, honey, and wine so they are ready to be added quickly.
When the shallots are softened add the minced garlic. Sauté for another minute and then add both the red wine vinegar and honey. Stir together, scraping up the browned bits from the pan. Add the red wine and chicken broth.
Return the chicken to the braiser along with any juices. Add half of the grapes to the pan along with the sprigs of fresh rosemary. Cover and bake for 30 to 40 minutes until the chicken is fully cooked and tender.
Remove the chicken to a serving platter. Scoop out the cooked grapes and rosemary with a slotted spoon and discard. Bring the sauce to a simmer and allow to reduce slightly. Add the remaining grapes. Taste and adjust the flavour if needed. You may want to add a splash of red wine vinegar or a drizzle of honey to create a balanced sweet and tangy flavour. Season with kosher salt and pepper.
Make a slurry with the cornstarch and water. Add to the cooking liquid and simmer until slightly thickened. Return the chicken to the brasier and nestle in the sauce and grapes. Serve the chicken with the sauce and grapes and spooned over. Enjoy!