Prepare Oven and Pan: Preheat your oven to 390°F (199°C). Line a large baking sheet with parchment paper.
Position Chicken: Arrange the chicken breasts on one side of the prepared baking sheet, leaving space for the asparagus.
Mix Zesty Marinade: In a small bowl, whisk together the Herbs de Provence, minced garlic, onion powder, olive oil, lemon zest, lemon juice, sea salt, black pepper, and optional red pepper flakes until well combined.
First Bake for Chicken: Spoon about half of the marinade evenly over the chicken breasts. Place the sheet pan in the preheated oven and roast for 22-25 minutes, or until the chicken is mostly cooked through.
Prepare Asparagus: While the chicken roasts, add the trimmed asparagus to the bowl with the remaining marinade. Toss to coat the asparagus thoroughly.
Combine and Broil: Carefully remove the sheet pan from the oven. Turn the oven setting to a low broil. Arrange the marinated asparagus around the chicken on the sheet pan. Drizzle any remaining marinade from the bowl over the chicken and asparagus. Place the thin lemon slices over and around the chicken and asparagus.
Finish Under Broiler: Return the sheet pan to the oven. Broil for 8-12 minutes, watching carefully to ensure the asparagus becomes tender-crisp and slightly charred, and the chicken is golden brown and cooked through (internal temperature reaches 165°F or 74°C).
Rest and Serve: Remove from the oven and let the chicken rest for a few minutes before serving immediately with the asparagus.