Cut the backbone out of the chicken with sharp knife or poultry sheers. Pull the chicken open.
Rinse in cold water. Pat dry. Salt chicken on both sides.
Mix lemon juice and mustard together well. Add olive oil garlic cloves, thyme, lemon juice.
Reserve enough of this to put on the cauliflower, before marinating chicken.
Put chicken in marinade for one hour at room temperature. You can put chicken in marinade overnight, but take out an hour before roasting.
Preheat oven to 450 degrees F.
Cut core out of cauliflower, leaving it intact and whole. Some pieces may come off, and you'll reserve those to bake during the last 20 minutes.
In a shallow baking dish, place cauliflower, doused with the marinade, salt and pepper.
Place chicken on top of cauliflower and roast. You have an option of baking both separately. Placing chicken on top of cauliflower will give cauliflower extra flavor from the chicken juices.
Bake for 20 minutes. Reduce oven to 400.
At this point, take chicken off cauliflower and let them roast separately.
Chicken should be done in 40 minutes; however use a meat thermometer and remove from oven at 165 degrees.
Let chicken rest for 15 minutes before cutting.
Serve with cauliflower and pan juices.