(can be done easily in food processor to save the dicing)
Preheat over to 425 degrees.
Place chicken breast-side down with the wings/ neck facing towards you. Use kitchen scissors to cut through the ribs right along the spine on both sides to remove backbone from end to end.
Open the rib cage and use a heavy chefs knife to score down the sternum. This will help to open out the breast bone and flatten the chicken.
Season the under side of the chicken with about 1/2 tsp salt and a pinch of black pepper.
Flip the chicken over with the skin-side up and place on a baking sheet or baking pan.
Using the palm of your hand, push firmly over the breast bone so the chicken lays completely flat. You want it to lay flat so all parts of the chicken bake evenly. Separate the skin from the meat over breast, thigh and drumstick areas by inserting your fingers under the skin at the top of the breast. As much as possible, it's important to keep the skin attached to the chicken except at that entry point.
In a small mixing bowl combine the flavored butter ingredients and mash together with a fork for a couple of minutes until most of the lemon juice is incorporated into the butter. (This can be done easily in a food processor.)
Spread 2/3 of it under the chicken skin and spread and dot remaining butter all over the chicken.
Cut potatoes into halves or quarters depending on size
Cut carrots into 1 inch pieces
Cut brussell sprouts in half and place cut side down
Spread evenly around chicken in baking pan.
Roast 45 minutes, basting the chicken and vegetable with pan juices, every 20 mins.
Chicken is done with thickest part of breast registers 160 degrees.
Let rest 10 mins (uncovered) before serving.