Preheat oven to 425°F. Line a large sheet pan with parchment or foil (optional).
Toss potatoes with 3 tbsp olive oil, 1 tsp Italian seasoning, garlic powder, 1 tsp salt, and ½ tsp pepper. Spread on the pan. Roast 15 minutes.
Pat chicken thighs dry. Season with remaining Italian seasoning, paprika (optional), ½ tsp salt, and ½ tsp pepper.
Remove pan, push potatoes to the sides, place chicken in the center skin-side up. Roast 25 minutes.
Toss green beans with 2 tbsp olive oil, ¼ tsp salt, ¼ tsp pepper, and a pinch of Italian seasoning.
Remove pan, scatter green beans around chicken and potatoes. Roast 10–12 minutes, until chicken reaches 165°F and potatoes are tender.
Squeeze lemon over everything. Sprinkle with parmesan (optional). Stir in fresh garlic (optional). Rest 5 minutes, then serve.