Make the bechamel; place onion and butter in a medium saucepan over medium heat, and cook until onion is translucent and softened, about 5 minutes. Add flour, and cook, stirring constantly, for 2 minutes. Whisk in milk in a slow, steady stream. Cook, stirring constantly, until mixture has thickened, about 4 minutes. Add 1 cup Gruyere, and stir until cheese melts. Remove from heat, add nutmeg, and season with salt and pepper.
To assemble; Heat oil in a large saute pan over medium heat. Add garlic, and cook, stirring occasionally, until wilted, about 2 minutes. Add red-pepper flakes, and season with salt, pepper, and a squeeze of lemon. let cool slightly. Squeeze out any excess liquid, and coarsely chop.
Preheat oven to 450°F
Reserve 2 tablespoons of bechamel. Mix remaining bechamel and spinach in a medium bowl. Butter and rimmed baking sheet. Place 1 crepe on center of sheet and top with 3 tablespoons spinach mixture, spreading evenly. Top with another crepe, then 3 tablespoons spinach mixture. Repeat with remaining crepes and spinach mixture, ending with a crepe. Spread reserved bechamel on top and sprinkle with remaining 1/4 cup cheese.
Bake until cheese melts and turns golden brown, 10 to 12 minutes. Let cool slightly, then transfer gateau de crepes to a platter. Cut into wedges and serve warm.