Preheat oven to 350°F (180°C). Place spinach in large colander in sink. Bring 6 cups (1.5 L) water to boil in a kettle. Pour boiling water over spinach, drain and cool.
Meanwhile, in large saucepan, make white sauce by melting butter over medium heat; whisk in flour, salt and pepper. Cook, whisking constantly, 1 to 2 min., or until mixture turns pale golden and is texture of scrambled eggs. Gradually whisk in milk until sauce is smooth. Bring just to a boil then immediately reduce heat to low. Simmer 5 to 7 min., stirring often, until sauce thickens. Remove from heat. Whisk in nutmeg.
Arrange a layer of lasagna noodles in the bottom of lightly greased 13 x 9-in. (3 L) baking dish, cutting noodles to size if necessary. Top with about 3/4 cup (175 mL) white sauce and ¼ cup (60 mL) each mozzarella and Parmesan. Arrange ⅓ of the zucchini strips over the cheese. Cover zucchini layer with ⅓ of the spinach. Repeat layering process two more times, finishing with a layer of lasagna noodles topped with remaining white sauce and cheese. Bake until lasagna is golden brown and bubbling, about 45 to 50 min. Let stand 10 min. before slicing.
Substitute thinly sliced eggplant (2 small eggplants) for zucchini strips.