Sumac Chicken with Cauliflower and Brussels Sprouts

Name: Sumac Chicken with Cauliflower and Brussels Sprouts
Ingredients: 3 tablespoons olive or avocado oil, divided; 1 tablespoon Sumac; 1 teaspoon kosher salt, divided; 1 teaspoon light brown sugar; 1 teaspoon paprika; 1/4 tsp red pepper flakes; 1 pound cauliflower florets; 1 pound Brussels sprouts, halved lengthwise; 2 pounds chicken thighs/drumsticks/bone-in, skin-on breast; 1 small lemon, halved lengthwise and thinly sliced; 1 small red onion, cut into ¾ inch wedges; 1 cup cup finely chopped fresh flat-leaf parsley or cilantro; preferably, a mix; 1 tablespoon fresh lemon juice; 1 small garlic clove