Pour buttermilk into a large bowl. Add seasonings and stir. Add chicken. Cover and refrigerate 4-6 hours (can soak longer).
Wash and prepare vegetables. You can slice the baby potatoes and brussels sprouts in half if you'd like.
Arrange vegetables onto a large baking sheet that has been sprayed with non stick spray.
Arrange chicken legs amongst vegetables.
Drizzle everything in olive oil and sprinkle with kosher salt and pepper.
Bake at 400 degrees for 1 hour. Check to make sure vegetables are tender and chicken is at least 165 degrees.
Video
Notes
Ideally everything will be in one layer, but if you are like me, and making this for a crowd, you can place the chicken legs on top of the vegetables and roast for an additional 10-15 minutes to make sure the vegetable under the chicken are cooked through.