On a baking sheet tray, place all lemon slices, while reserving 2 slices. Top with garlic cloves and several sprigs of thyme. Set aside.
On a cutting board, add chicken breast side down. With a knife, cut out the backbone of the chicken. Reserve backbone for broth (can freeze until ready to use). Next, flip chicken over, breast side up. Press firmly on breastbone to flatten. Salt and pepper the underside of the chicken.
Next, on the baking sheet with the lemons, take the chicken and place breast side down on top of lemon slices. Rub chicken with butter over and under the skin. Season with more salt and pepper, all over. Slide 1 lemon slice under skin on each breast. Place remaining thyme sprigs on top of chicken. (Before next step, don’t forget to wash your hands!)
Arrange Brussel sprouts on each side of the chicken. Drizzle with melted butter and sprinkle with more salt and pepper.
Roast for 45 to 55 minutes or more. Brussel sprouts should be nice and crispy. Let rest 10 minutes, before slicing.
Slice chicken and plate. Add Brussel sprouts with lemon slices and spoon on top a bit of juice from the baking tray. Enjoy!