Pre-heat the oven to 425°F.
In a large bowl, toss together the chicken with 1 tablespoon olive oil, garlic and za’atar, if using; season well with salt and pepper.
Place the shallots and the grapes on the sheet pan and gently toss with the remaining olive oil; season well with salt.
Nestle the chicken, skin-side up, in between the shallots and grapes and lay the thyme sprigs on top of the mixture.
Place in the pre-heated oven and roast for 25 to 30 minutes or until the chicken is cooked through and the shallots and grapes begin to soften and caramelize.
Remove the chicken from the oven; set aside.
Turn the oven to broil and move the oven rack to sit right below it.
Remove and discard the thyme sprigs; place chicken under the broiler for 1 to 2 minutes until the skin of the chicken is crispy and golden.
Remove from the oven and scatter with chopped thyme; season with flaky salt.