Preheat the oven to 425°F (220°C).
In a large bowl, combine the halved baby potatoes, 1 tablespoon olive oil, half of the minced garlic, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Toss to coat evenly.
Spread the potatoes on a large rimmed baking sheet in a single layer. Roast in the oven for 15 minutes.
While the potatoes roast, in the same large bowl, combine the chicken thighs, remaining 2 tablespoons olive oil, lemon juice from 1 lemon, remaining minced garlic, dried oregano, remaining salt and pepper, and crushed red pepper flakes. Toss to coat the chicken evenly.
Remove the baking sheet from the oven. Add the green beans and lemon slices around the potatoes.
Place the chicken thighs skin-side up on the baking sheet, arranging them evenly among the vegetables.
Return the baking sheet to the oven and roast for an additional 20 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and the potatoes are tender.
If desired, broil for 2-3 minutes to crisp the chicken skin, watching carefully to prevent burning.
Remove from the oven and let rest for 5 minutes before serving.