1/2 of a small onion, sliced thinly (about half cup)
Preheat oven to 400 degrees F.
In a small bowl, whisk the lemon zest, juice, garlic, oil, oregano, sugar, salt, and pepper.
Place chicken (skin-side up) in a greased glass baking dish. Arrange the bell pepper, tomato, and onion all around the pieces of chicken. Pour the lemon mixture evenly over the top. Cover dish with aluminum foil and bake for 20 minutes. Remove the foil and spoon the pan juices repeatedly over the chicken to moisten.
Bake uncovered, basting every 10 minutes, for about 30 more minutes. Serve chicken and vegetables with all of the pan juices.