Cook the spinach, covered, in boiling salted water until tender, about 4 minutes. Rinse under cold running water; drain, pressing out all liquid with the back of a spoon. Chop fine.
In a medium saucepan over low heat, melt 2-1/2 T. of the butter. Stir in flour. Cook, stirring constantly, 2 minutes. Whisk in hot milk. Cook until thick, about 2 minutes. Add gruyere cheese, the 2 T. finely grated parmesan, salt, pepper, nutmeg, and hot pepper sauce. Stir in spinach; remove from heat. Stir in lemon zest; mix thoroughly. Transfer to a shallow, buttered, 4-cup baking dish.
In a small skillet over medium heat, melt remaining 2 T. butter. Stir in bread crumbs. Cook, stirring constantly, until golden. Spoon crumbs evenly over spinach mixture. Sprinkle with 1/4 cup of roughly grated parmesan.
Bake in a preheated 425-degree oven for 14 minutes.