Preheat oven to 450 degrees F. Line a large shallow baking pan with foil, if desired; set aside. In a small bowl combine mustard, honey, parsley, salt, and pepper. Pat chicken dry with paper towels. Brush mustard mixture evenly over chicken pieces. Arrange chicken in prepared baking pan. Add cauliflower, brussels sprouts and lemon slices to pan (divide mixture between two baking pans, if crowded). Drizzle all with oil.
Roast, uncovered, about 40 minutes, stirring vegetables once, or until chicken breast registers 165 degrees F in the thickest part and legs and thighs register at least 170 degrees F and vegetables are tender. (Cover with foil the last 10 minutes of of roasting to prevent over browning, if necessary.) Garnish with additional parsley, if desired and drizzle with pan juices.