Kosher salt and freshly ground black pepper
8 ounces lasagna noodles (about 12)
3 tablespoons olive oil, plus more for tossing and greasing the baking dish
2 pounds bulk breakfast sausage, thawed
1 bunch scallions, whites cut into 1/2-inch pieces and greens thinly sliced
1/2 cup all-purpose flour
4 cups whole milk
1/2 teaspoon freshly grated nutmeg
2 cups shredded low-fat mozzarella cheese
8 ounces white Cheddar, shredded
1/2 cup grated Parmesan
Two 10-ounce packages frozen chopped spinach, thawed and excess liquid squeezed out
1/4 cup fresh parsley leaves, chopped