Preheat oven to 350 degrees F and set out a seasoned 12-inch cast iron skillet. You can also do this in a 9x13 casserole dish if you don't have a cast iron pan.
Prep your Cornish hens by removing giblets and patting the hen dry so the seaosning will stick.
Season the outside of the Cornish hens with 1 teaspoon kosher salt and 1/2 teaspoon black pepper. Stuff the hens with 2 pieces of lemon, 3 pieces thyme and 2 pieces rosemary. It doesn't have to be perfect just stuff it into the cavity until it fits.
Place the hens in the skillet.
In a small bowl add fingerling potatoes and drizzle with 1 tablespoon olive oil, 1/2 teaspoon kosher salt, and 1/4 teaspoon black pepper. Take the remaining sprig of rosemary and mince the leaves. Add to the potatoes and toss to evenly coat.
Add the potatoes to the pan with the Cornish hens. Make sure to place th eptoatoes around the hens.
Add to the oven and roast for 30 minutes.
While the hens are roasting add the Brussels sprouts to the bowl the potatoes were in.
Drizzle the sprouts with the remaining tablespoon of olive oil, kosher salt, and black pepper. Toss to coat evenly.
After 30 minutes of roasted, remove the skillet from the oven, and add the Brussels sprouts to the pan surrounding the Cornish hen. Gently stir to incorpate the sprouts with the potatoes.
Glaze the Cornish hens with cranberry BBQ sauce.
Add back to the oven and bake for 20-25 minutes or until the Brussels sprouts are fully roasted, the potatoes are soft and the chicken has reached an internal temperature 160 degrees F.
Remove from the oven and serve with leftover BBQ sauce.
Note: If you buy larger cornish hens you will have to adjust the cooking time. You will want to roast your hens longer and then add in the brussels at the end so they do not overcook.