In a large mixing bowl, combine the chicken thighs with Italian seasoning, paprika, olive oil, lemon juice, lemon zest, Dijon mustard, minced garlic, and brown sugar. Ensure the chicken is evenly coated in the marinade for the best flavor. Let it marinate for at least 30 minutes; this step is crucial for enhancing the taste of your Herbed Sheet Pan Chicken Thighs.

Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper or a silicone mat to prevent sticking and make cleanup easier.

In a separate bowl, whisk together the remaining lemon juice, olive oil, salt, and pepper. Add the cremini mushrooms and sliced red onion, tossing to coat them in the mixture. Let the vegetables marinate for about 10-15 minutes.

Spread the marinated chicken thighs evenly across the prepared baking sheet, leaving some space between each piece. Scatter the marinated onions and mushrooms around the chicken.

Place the baking sheet in the preheated oven and bake for 30-35 minutes. Keep an eye on the chicken; it’s ready when the skin is crispy and golden brown and the internal temperature reaches 165°F (75°C).

Once cooked, remove the baking sheet from the oven and let the Herbed Sheet Pan Chicken Thighs rest for about 5 minutes. Serve the chicken with the roasted vegetables.