Heat oven to 450°F.
On a large rimmed baking sheet, toss cauliflower with 2 tablespoons oil, then 2 teaspoons garam masala.
Add jalapeños, onion, and ½ teaspoon each salt and pepper and toss to combine.
Arrange in a single layer; roast 10 minutes.
Meanwhile, in a medium bowl, combine brown sugar, cayenne, and remaining tablespoon olive oil, 2 teaspoons garam masala, and ½ teaspoon each salt and pepper.
Add chicken and turn to coat.
Nestle chicken among vegetables, reduce oven temperature to 400°F, and roast until chicken is cooked through and vegetables are golden brown and tender, 15 to 20 minutes.
Sprinkle with cilantro and serve with lemon wedges if desired.