1. Preheat oven to 450 degrees. On a rimmed baking sheet, toss onion, sweet potatoes, brussels sprouts, and 3 thyme sprigs with 2 tablespoons oil. Season with salt and pepper; spread in a single layer. Season chicken cavity with salt and pepper, then stuff with lemons and remaining 6 thyme sprigs. Rub remaining 1 tablespoon oil over skin, brush with mustard, and season with salt and pepper. Tie legs with twine. Place on top of vegetables; roast 20 minutes.

2. Reduce temperature to 375 degrees and roast until a thermometer inserted into thickest part of thigh (avoiding bone) registers 165 degrees, 40 to 45 minutes more. Transfer chicken to a carving board; let stand 15 minutes. Meanwhile, return vegetables to oven. Continue roasting until tender and golden brown in places, 12 to 15 minutes more. Carve chicken and serve, with vegetables and lemon wedges alongside.