How to Make It
Heat oven to 350° F. Heat the oil in a large skillet over medium-high heat. Add the sausage and cook, breaking it up with a spoon, until browned, 6 to 8 minutes. Using a slotted spoon, transfer the sausage to a large bowl.
Add the butter, onions, celery, garlic, sage, ¾ teaspoon salt, and ½ teaspoon pepper to the skillet. Cook, stirring often, until the vegetables are tender, 8 to 10 minutes.
Toss the vegetables, bread, broth, walnuts, eggs, and parsley with the sausage. Transfer to a 9-by-13-inch baking dish and cover with foil. Bake until heated through, 30 to 40 minutes. Uncover and bake until browned, 15 to 20 minutes more.