In 3-quart saucepan, heat 1 inch water to boiling. Add spinach. Cook about 2 minutes or until wilted. Immediately remove spinach to a bowl of ice water. Drain; squeeze out excess water. In 10-inch skillet, heat olive oil over medium-high heat. Add garlic to oil; cook until lightly browned. Add drained spinach; cook about 2 minutes or until wilted.
In small bowl, combine the ricotta, egg yolk, and nutmeg; set aside.
To make polenta, in 2-quart saucepan, heat broth, salt, and roasted garlic oil to boiling. Gradually whisk in the polenta; reduce heat to low. Cook 25 to 30 minutes, stirring frequently, until the grains are soft. Remove from heat; stir in 1 1/3 cups Parmesan cheese.
Reserve 2 tablespoons of the marinara sauce for the top of the gratin.
Heat oven to 375°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray. Spread half of the polenta in baking dish. Top polenta by layering with half the marinara sauce, 2 tablespoons Parmesan cheese, 2 tablespoons panko bread crumbs and half each of the mozzarella cheese and spinach mixture; repeat.
Spoon ricotta mixture in small dollops over top of spinach mixture. Spoon reserved 2 tablespoons marinara sauce over gratin. Bake uncovered 45 to 50 minutes until bubbling and heated through to center. Allow to stand 5 minutes before serving.