Preheat and prep: Heat the oven to 425°F (220°C).
Line a large sheet pan with foil or parchment for easier cleanup. Pat the chicken thighs dry with paper towels to help the skin crisp.
Make the garlic herb sauce: In a small bowl, whisk together melted butter, 2 tablespoons olive oil, minced garlic, lemon zest, 1 tablespoon lemon juice, Dijon (if using), smoked paprika, Italian seasoning, thyme, half the parsley, salt, pepper, and a pinch of red pepper flakes.
Season the chicken: Place the chicken on the sheet pan. Loosen the skin slightly with your fingers and spoon some sauce under the skin.
Brush more sauce on top and around each thigh. Reserve 1–2 tablespoons of the sauce for the asparagus.
Roast the chicken: Bake for 20 minutes. The skin should start to brown and the pan juices will sizzle.
Prep the asparagus: While the chicken roasts, toss asparagus with the remaining 1 tablespoon olive oil, a pinch of salt and pepper, and the reserved garlic herb sauce.
Add the asparagus: After 20 minutes, remove the pan.
Scatter the asparagus around the chicken in a single layer. Return to the oven.
Finish roasting: Bake another 12–18 minutes, until the chicken reaches an internal temperature of 175–185°F at the thickest part (thigh meat is most tender a bit higher than 165°F) and the asparagus is tender-crisp with slight char.
Optional broil: For extra-crispy skin, broil the chicken for 1–2 minutes at the end. Watch closely to avoid burning.
Rest and serve: Let the chicken rest 5 minutes.
Squeeze the remaining lemon juice over the chicken and asparagus. Sprinkle with the rest of the parsley. Taste and adjust salt and pepper as needed.
Serve with the pan juices spooned over the top.