Preheat the oven to 400 degrees F (200 degrees C).
To spatchcock chicken: Place chicken spine-up and breast-down on a cutting board. Make a small 1/4- to 1/2-inch cut at the tip of the breast bone where the two breasts meet, using a very sharp and strong pair of kitchen shears. Flip the bird over, breast-side up. Press down on the bird so that it lays flat.
Place chicken skin-side down on a sheet or roasting pan and season with salt and pepper.
Lift skin away from the meat gently by sliding your fingers into the pockets at the end of the chicken breasts, thighs, or both, trying not to tear. Slide lemon slices, garlic butter gently under the skin, as far back as you can, having at least 2 slices for each thighs and each breast. Slide rosemary sprigs carefully under the skin, folding in half to avoid tough stems if necessary. Season chicken with additional salt and pepper, if desired. Place Quartered Onion evenly under the chicken.
Roast in the preheated oven until an instant-read thermometer inserted into the dark meat near the bone reads at least 165 degrees F (74 degrees C), about 50 minutes.
Remove from the oven and let sit for 5 minutes to allow juices to settle before carving.
Serve with herbed mash potatoes, creamed spinach and side salad with vinaigrette.
The Spatchcocking part, it’s a breeze. And because you’ve increased the surface area, it cooks a lot faster!”