Preheat the oven to 425°F (220°C).
In a large bowl, toss the halved Brussels sprouts with 1 tablespoon olive oil, 1/4 teaspoon salt, and 1/4 teaspoon black pepper until evenly coated.
Place the Brussels sprouts on a large sheet pan in a single layer, leaving space for the chicken thighs.
In the same bowl, combine the remaining 1 tablespoon olive oil, garlic powder, smoked paprika, 1/4 teaspoon salt, 1/4 teaspoon black pepper, and crushed red pepper flakes.
Pat the chicken thighs dry with paper towels, then rub the spice mixture evenly over the skin and under the skin of each thigh.
Arrange the chicken thighs skin-side up on the sheet pan next to the Brussels sprouts.
Roast in the preheated oven for 30 to 35 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and the Brussels sprouts are tender and caramelized.
For extra crispy skin, switch the oven to broil and broil the chicken and Brussels sprouts for 2 to 3 minutes, watching carefully to avoid burning.
Remove from the oven and squeeze fresh lemon wedges over the chicken and Brussels sprouts before serving.