Pat the chicken dry and place in a large bowl or large ziptop bag.
In a small bowl, combine the garlic, 2 tablespoons oil, paprika, Italian seasoning, lemon zest and juice, 1 teaspoon salt, and 1/4 teaspoon pepper.
Pour over the chicken and turn to coat the chicken evenly.
Let sit at room temperature for at least 30 minutes or cover and refrigerate for up to 12 hours.
When ready to bake, preheat the oven to 425°F and place one rack in the center and one in the upper third.
Line a baking sheet with parchment paper and place the onions in the center.
Drizzle the onions with the remaining 1 tablespoon oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Toss to coat.
Remove the chicken from the marinade; shake off and discard any excess marinade.
Clear some spaces in the onions for the chicken thighs, then nestle them in, spacing them evenly throughout the pan.
Bake the chicken thighs for 15 minutes.
Remove the pan and toss the onions to promote even cooking.
Return to the oven and bake for 10 to 20 additional minutes, until the internal temperature of the chicken reaches 165°F.
Let rest on the pan for 5 minutes.
Stir the onions once more, then place the rack on the upper third rack and broil for 1 to 2 minutes to crisp the chicken skin.
Sprinkle with parsley and serve hot, with the yummy, jammy onions complete with juices spooned alongside.
