In a large bowl, combine olive oil, minced garlic, lemon juice, chopped rosemary, chopped thyme, kosher salt, and black pepper to create the marinade.
Add the chicken thighs to the marinade, turning to coat evenly. Cover and refrigerate for at least 1 hour, or up to 4 hours for best flavor.
Preheat the oven to 425°F (220°C).
In a separate bowl, toss baby potatoes, red bell pepper, yellow bell pepper, and red onion with 2 tablespoons olive oil, 1/2 teaspoon kosher salt, and 1/4 teaspoon black pepper.
Spread the vegetables in a single layer on a large baking sheet.
Remove the chicken thighs from the marinade, letting excess drip off, and place them skin-side up on another baking sheet.
Roast the vegetables in the preheated oven for 25 minutes.
While the vegetables roast, heat a large skillet over medium-high heat. Place the chicken thighs skin-side down and sear for 5-7 minutes until the skin is golden brown and crispy.
Flip the chicken thighs and sear the other side for 3 minutes.
Transfer the seared chicken thighs to the oven and roast alongside the vegetables for 15-20 minutes, or until the internal temperature reaches 165°F (74°C).
Remove chicken and vegetables from the oven. Let the chicken rest for 5 minutes before serving.
Serve the marinated chicken thighs with the roasted vegetables on the side.