Preheat oven to 450 degrees. Cut onions into 3/4 inch thick wedges and place in cast iron skillet or roasting pan. Cut garlic heads in half and place in skillet nestled with onions. Drizzle 3 tablespoons olive oil over the onions and garlic, sprinkle with about a teaspoon of salt and toss gently. Adjust garlic heads to they are cut side down in the pan.
In a small bowl combine 1.5 teaspoon salt** (I used a Kosher chicken so cut back quite a bit on the salt, Molly uses 4 teaspoons in her recipe), 1 tablespoon paprika, 1 tablespoon brown sugar, 1 teaspoon black pepper and 2 tablespoons olive oil mixing until a paste forms. Place the chicken breast side up on top of the onions in the skillet. Rub the paste all over the chicken – really get into the nooks and crannies and underside of the chicken!
Roast the chicken for 50-65 minutes, giving the onions a toss after 30 minutes. My bird got a bit too dark, you can tent with foil if it starts to get too dark for the remainder of the cook time if this happens to you.
Sprinkle the onions and garlic with the dill. Carve chicken and return on top of the onions and garlic for serving. Slather with that dijon for maximum pastrami sandwich vibes.