Place chicken thighs skin side up on an 18x13-inch rimmed baking sheet. Sprinkle with 2 teaspoons salt. In a small bowl, whisk together 2 tablespoons oil, 2 tablespoons mustard, 1 tablespoon garlic, and zest. Brush mustard mixture onto chicken. Bake for 10 minutes.
In a large bowl, combine 1 tablespoon garlic, 1 tablespoon thyme, remaining 2 tablespoons oil, and remaining 1 teaspoon salt. Add carrots, broccoli, and cauliflower, tossing to coat. Arrange vegetables and lemon halves around chicken in pan.
Bake until chicken is cooked through and vegetables are tender, about 20 minutes.
In a small saucepan, melt 2 tablespoons butter over medium heat. Add onion and remaining 1 tablespoon garlic; cook, stirring constantly, until tender. Add wine and remaining 1 tablespoon thyme; cook, stirring constantly, until wine is reduced by half, 1 to 2 minutes. Whisk in remaining 2 tablespoons mustard and remaining 1 tablespoon butter; remove from heat. Serve sauce with chicken. Garnish with thyme, if desired.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/dijion-mustard-chicken-vegetables/