Preheat oven to 400*. Rinse spinach and place in a saucepan with water still clinging to its leaves. Cover and cook for 5 minutes or until wilted. Drain, refresh under cold water, squeeze out all excess liquid. Chop finely. Cook lasagna sheets in plenty of boiling salted water, until tender but still firm to the bite, drain, and pat dry with kitchen towels.
Brush a 12x8-inch ovenproof dish with a little of the melted butter. In a bowl, combine the spinach, Fontina (or Gruyere), Parmesan, Ricotta, cream cheese, nutmeg, salt and pepper.
Cover the base of the dish with a later of lasagna sheets and spread with 3 tablespoons of the spinach mixture. Repeat layering until all the lasagna is used, making no more than 6 layers. Finish with the remaining spinach mixture. Drizzle with the remaining melted butter, then cook for 20-30 minutes, until golden brown.