Heat the oven to 425°F. Line a large rimmed sheet pan with parchment (trust me on the parchment).
Toss the potatoes with half the olive oil, half the garlic, half the Italian seasoning, and half the salt right on the pan. Spread in a single layer and roast 15 minutes.
Push the potatoes to the edges. Add the chicken thighs skin-side up, then scatter the green beans and tomatoes around them. Drizzle everything with the remaining olive oil, garlic, seasoning, salt, pepper, and lemon juice. Tuck the lemon slices around the pan.
Roast 25 to 30 minutes, until the chicken hits 165°F internally (USDA) and the skin is crisp and golden.
Rest 5 minutes, then squeeze the roasted lemon over everything.