Grain Stuffing with Sausage

Name: Grain Stuffing with Sausage
Ingredients: 3/4 cup farro, rinsed; 3/4 cup rye berries, rinsed; 5 cups low-sodium chicken broth; 5 sprigs thyme, plus 1 tablespoon chopped leaves; 2 bay leaves; 3/4 cup millet, rinsed; 1 medium-large celery root, peeled and cut into 1/2-inch cubes; 3 red onions, cut into 1/2-inch pieces; 4 sweet potatoes, peeled and cut into 1/2-inch pieces; 3 tablespoons extra-virgin olive oil; 2 tablespoons chopped fresh sage; Kosher salt and freshly ground pepper; 4 tablespoons unsalted butter, plus more for the baking dish; 1 pound turkey or chicken sausage, casings removed, meat crumbled; 1 cup shredded whole-milk mozzarella cheese (about 4 ounces); 1/2 cup grated parmesan cheese