Prepare Ingredients: Pat the chicken thighs dry with paper towels after ensuring they are clean and any large pieces of excess fat are trimmed. In a large mixing bowl, combine the chicken thighs, halved Brussels sprouts, sliced Italian sausage, thinly sliced shallots, garlic slices, and lemon slices.
Season and Marinate: Coat the chicken and vegetable mixture with the olive oil. Generously sprinkle with the crushed rosemary, thyme, rubbed sage, fennel seeds, smoked paprika, sea salt, and black pepper. Use your hands to gently toss all ingredients, ensuring an even distribution of oil and seasonings. Let the mixture rest at room temperature for approximately 20 minutes, allowing the flavors to begin melding.
Preheat Oven: Position an oven rack in the center of the oven and preheat to 450∘F(230∘C). Lightly spray a large, rimmed baking sheet with cooking spray.
Arrange for Roasting: Transfer the seasoned chicken and vegetable mixture to the prepared baking sheet, arranging everything in a single, even layer. Ensure the chicken thighs are placed skin-side up on top of the vegetables to promote crispy skin.
Roast: Place the baking sheet in the preheated oven. Roast for 28 to 35 minutes. The chicken should be cooked through, with the skin browned and crispy, and an internal temperature of 165∘F(74∘C) when checked with a meat thermometer inserted into the thickest part near the bone (avoiding contact with the bone). The Brussels sprouts should be tender and have some caramelized edges.
Rest and Serve: Carefully remove the baking sheet from the oven. Allow the roasted chicken and vegetables to rest on the pan for 5 minutes before serving. This resting period helps the chicken retain its juices, making it more succulent.