Preheat the oven to 425 degrees F.
Season the chicken thighs on both sides with 1 teaspoon salt and 1/2 teaspoon pepper.
Heat the olive oil in a roasting pan on the stovetop over medium-high heat. Add the chicken, skin-side down, and sear until golden brown, 4 to 5 minutes; remove to a plate.
Add the potatoes, garlic, onions, thyme, parsnips, 1 teaspoon salt and 1/2 teaspoon pepper to the roasting pan; stir. Cook until the vegetables are warmed and coated in oil, 2 minutes.
Place the chicken, skin-side up, on top of the vegetables. Scatter with the lemon slices and thyme sprigs.
Roast until the chicken is cooked through and the potatoes are tender, 35 to 40 minutes.