Chicken Sausage with Spinach and Artichoke Lasagna

Name: Chicken Sausage with Spinach and Artichoke Lasagna
Ingredients: 8 tablespoons (1 stick) unsalted butter; 1 cup finely chopped yellow onion; 3 tablespoons minced garlic; 1/2 cup all-purpose flour; 7 cups milk; 1 1/2 teaspoons kosher salt; 1/2 teaspoon freshly ground black pepper; 1/4 teaspoon freshly grated nutmeg; 2 (10 oz.) boxes or bags frozen chopped organic spinach, defrosted and wrung dry; 3 cups grated Parmesan; 2 tablespoons olive oil, plus more for coating baking dish; 1 1/2 lbs. chicken sausage; 2 (14-ounce) cans quartered artichoke hearts, drained and chopped; 1 pound oven-ready Lasagna sheets; 1 tablespoon butter, cut into 8 pieces