Preheat oven to 180°C (350°F).

Place spinach in a large colander in the sink. Pour 1.5 L (6 cups) boiling water over spinach. Drain and let cool.

Meanwhile, in a large saucepan set over medium heat, melt butter. Add flour, salt and pepper. Cook, whisking constantly, for 1–2 minutes or until mixture browns slightly and has the texture of scrambled eggs.

Gradually add milk and whisk until smooth. Bring to a boil, then immediately reduce heat to low. Simmer for 5–7 minutes, stirring often, until sauce thickens. Remove from heat and whisk in nutmeg.

Arrange a layer of lasagna noodles on the bottom of a lightly greased 3-L (13 in. x 9 in.) baking dish, cutting to size if necessary. Top with about 175 mL (3/4 cup) bechamel and 60 mL (1/4 cup) each mozzarella and Parmesan. Place one third of the zucchini strips on top of cheese. Cover zucchini layer with one third of the spinach.

Repeat the layering process twice more, ending with a pasta layer topped with remaining bechamel and cheese.

Bake until golden and bubbling, about 45–50 minutes. Let stand 10 minutes before serving.