Slice about half of the grapes in half lengthwise, or slice all of them if you prefer, and thinly slice the shallots. Set aside.
In a large bowl, whisk together the olive oil, balsamic vinegar, Italian seasoning, crushed garlic, salt, and pepper.
Add the sliced grapes and shallots to the bowl. Pat the chicken thighs dry with paper towels, sprinkle with a pinch of salt and pepper, then add them to the bowl.
Toss everything thoroughly until the chicken, grapes, and shallots are evenly coated with the balsamic mixture.
Arrange the chicken, grapes, and shallots on a large sheet pan lined with parchment paper, making sure not to overcrowd the pan. Roast at 425°F for 22-26 minutes, or until the chicken reaches an internal temperature of 165°F in the thickest part, and the grapes and shallots are caramelized.
Let the chicken rest for a few minutes, then spoon the flavorful sheet pan juices over the chicken, grapes, and shallots before serving. Garnish with fresh parsley if desired, and enjoy!