Pre-heat your oven to 425 degrees and make sure you have your 3 oven racks in the middle of the oven.
Whisk the Maple Mustard Sauce ingredients together and set aside.
Split your chicken by cutting down each side of the spine to remove the backbone. (You can see how this looks in the picture above). Put the chicken breast side UP on a baking sheet, press the chest down flat, and place an onion round under each breast and leg. Put 1 cup of water on the baking sheet as well.
Loosen the skin of the breast using your fingers, and put about 2-3 tbsp of butter under the skin (see the above image). Brush a thin layer of the maple mustard sauce on the chicken (use about ¼ of what you made), and place in the 425 degree oven for 20 minutes.
While the chicken is cooking, you can get your veggies ready. Toss the veggies with the butter/salt/pepper on a large sheet pan, and then split the veggies between 2 sheet pans. If you leave them super piled up and crowded they won’t cook as fast and won’t get the yummy roasted flavor. Use 2 sheet pans. If you have a smaller crowd to feed, you can use less veggies and cover 1 sheet pan.
When the chicken has cooked for 20 minutes, take it out, brush on more of the maple mustard sauce, leaving about ¼ of it behind for one more coat at the end. Put the chicken back in the oven along with the veggie sheet pans, and roast everything for 30 minutes.
Take the chicken out, brush on the last of the Maple Mustard Sauce, give the veggies a quick stir, and roast everything for another 5 minutes.
Let the chicken rest for 5 minutes before cutting – you can leave the veggies in the oven to stay warm.