Spray a 9 x 13 casserole dish with non-stick cooking spray.
Lay the chicken breast down in the dish, I put them towards the corners.
Add the potatoes and spread evenly in the dish.
Pour the dressing over top of the chicken and potatoes.
Lay lemon slices over the chicken and potatoes randomly.
If you want rosemary you can add it now, or just use it for garnish later. Some folks like rosemary, some don't. So it's a completely optional ingredient.
Place the casserole dish in the preheated oven. Cook for 55 minutes total. (Check the chicken at 30 minutes to make sure there's enough liquid, and stir the potatoes around. If you need more liquid you can add some water at this point. I baste everything at this point to keep the chicken moist. Slide the casserole dish back in the oven and cook for another 15 minutes. Check again by pulling the dish out of the oven and baste again, and move the potatoes around a bit. If you need some water add some. Slide back in and cook for another 10 minutes.) The basting will really help. Don't be shy to add a little water if the dressing cooks off too quickly.
Remove from the oven. Discard rosemary.
Transfer everything to a big platter and serve family style, or just place the casserole dish on a heat resistant trivet on the table.
Serve with a big old lemon chicken loving smile!
You can add water or chicken broth as it cooks to keep it moist and prevent it from sticking to the pan. I added about 1/2 cup at the thirty minute mark. Just keep an eye on it and add as needed.