Pre-heat oven to 425 °F. Prepare a cast iron skillet or baking sheet by layering with thick slices of red onion.
Make a compound butter by softening (not melting) 6 Tablespoons of butter. Crush or finely chop garlic cloves. Wash rosemary, strip from the stem, roughly chop. Use a fork to mix butter, garlic, rosemary, salt, and pepper with the juice from ½ of a lemon.
Remove chicken from package. Be sure to remove giblets or neck from the body cavity.
Using paper towels, pat the skin dry. Be sure to get the bottom of the chicken as well. Place the chicken on top of the thick slices of onions in your pan.
Rub down the inside of the chicken with a small amount of compound butter, then spread the remaining butter evening over the chicken. Make sure to the compound butter on the underside and all around the legs and wings.
Cut lemons into wedges and place inside the chicken to retain moisture.
Put chicken in the oven on a middle rack at 425 ° for 20 minutes.
Lower the temperature to 350° and finish cooking for about an hour and twenty minutes more.
The only way to know if a chicken is done is to get a thermometer and stick into the inner thigh near the breast. Find the meatiest part possible and measure from there. The finished temp should be 165°
Remove from oven and let rest, covered with foil for 10 minutes.
Carve and serve.