Cream Sauce: Melt butter over medium heat; cook onion for about 2 minutes or until softened. Sprinkle flour over top; cook, stirring, for 1 to 2 minutes or until incorporated. Slowly whisk in milk, cream, salt, pepper and nutmeg. Bring to boil, whisking constantly. Reduce heat; simmer for 3 to 5 minutes or until mixture is thickened. Set aside.
- Lasagna: Heat oil in large skillet set over medium heat; cook garlic for about 1 minute or until slightly softened. Stir in spinach, salt and pepper; cook for 3 to 4 minutes or until wilted. Let cool completely; squeeze out excess moisture.
- Preheat oven to 400˚F. Spoon 1/4 cup cream sauce into bottom of greased 8-inch square baking dish. Stir together ricotta, Parmesan and egg yolk. Lay one-third of the crackers in bottom of baking dish. Top with half of the ricotta mixture, half of the spinach mixture, one-quarter of the cream sauce and one-third of the mozzarella. Repeat layers once; layer remaining crackers, cream sauce and mozzarella over top.
- Bake for 25 to 30 minutes or until golden brown and bubbling. Let stand for 10 minutes before serving. Sprinkle with parsley.
Tip: To save time, substitute store-bought Alfredo sauce for cream sauce.