Make Stuffing: Heat pan over medium heat, and then add the olive oil, celery, onion, and garlic. Cook until onions and garlic are softened, about 1 minute. Add the sausage, breaking up the sausage with your spatula and cook until sausage is browned.
Add herbs (thyme, oregano, or rosemary), sage, parsley, salt and pepper. Cook for another minute until the spices are combined. Add the broth and then remove from heat.
Mix in the croutons or stuffing cubes and egg into the sausage mixture until everything is combined well. Set aside.
Roll & Bake Turkey: Preheat oven to 375°F.
Place the pounded turkey breast skin side down. Lay the stuffing on top of turkey breast. Fold one edge of turkey breast over to create a roll. Tie the turkey breast to hold the shape.
Season top of turkey breast roll with additional salt and pepper. Place the turkey breast roll in a baking dish (make sure the baking dish is large enough to fit the remaining stuffing - you won't add it yet, but will after the turkey breast roll bakes for a bit).
Roast the turkey breast roll for 45 minutes. Remove from oven and place the remaining stuffing around the turkey. Return to the oven and continue roasting for another 30 minutes, or until internal temperature of turkey reaches 165°F at turkey meat thickest part. Allow to rest for about 5-10 minutes, then slice and serve.