Set your oven to 375 degrees F to prepare for roasting the turkey roulade later.

In a large skillet over medium-high heat, cook the Italian sausage, breaking it into crumbles. Push the cooked sausage to one side. In the same skillet, melt 2 tablespoons of butter and sauté the diced onion and celery for about 5 minutes until softened. Combine the sausage and vegetables, then add the herb stuffing mix along with about 1 cup of chicken broth. Stir gradually until the mixture is moist, adding additional broth if needed.

Place the turkey breast flat on a cutting board. Using a sharp knife, slice horizontally through the middle of the breast, stopping about ¾ inch from the far edge to create a 'book' that opens flat. Cover with plastic wrap and pound the turkey with a meat mallet to an even thickness of about ½ inch. Season with kosher salt and black pepper, approximately ¼ teaspoon each.

Spread the prepared sausage stuffing evenly over the flattened turkey breast, leaving about a ⅓ inch border around the edges. Roll the turkey tightly from the longer side, ensuring the filling stays snug inside. Secure the roll with kitchen twine at intervals to keep its shape. Place the roulade seam-side down in a baking pan.

In a bowl, combine the softened butter with 1 teaspoon kosher salt, ½ teaspoon black pepper, and 1 teaspoon garlic powder. Rub half of this butter mixture all over the turkey roulade, reserving the rest for later basting.

Place the baking pan in the preheated oven and roast for 35 to 40 minutes at 375 degrees F. Then increase the oven temperature to 400 degrees F for the final 20 minutes. Just before the last 20 minutes of roasting, melt the remaining butter mixture and drizzle it over the roulade to baste and enhance browning. Continue roasting until a meat thermometer inserted into the thickest part reads 165 degrees F.

Remove the roulade from the oven and let it rest for 15 minutes. Carefully remove the kitchen twine, slice into rounds, and serve warm.