Preheat oven to 425°F and line a large sheet pan with parchment paper or aluminum foil.
In a small bowl, combine Cajun seasoning, garlic powder, onion powder, smoked paprika, salt, and black pepper.
Pat chicken thighs dry and rub with 1 tablespoon olive oil, then coat generously with the spice mixture; let sit for 10 minutes if time allows.
In a large bowl, toss halved potatoes with remaining 2 tablespoons olive oil and any leftover spice mixture.
Arrange potatoes in a single layer on the sheet pan, then nestle chicken thighs among the potatoes, skin-side up.
Roast for 30-35 minutes until chicken reaches 165°F internal temperature and potatoes are golden and crispy, then drizzle with lemon juice and garnish with fresh herbs before serving.