Preheat your oven to 425 degrees farenheit. Grease ramekins/baking dish with butter or cooking spray.
Cook Spinach Mixture: If using fresh spinach, wash it. In a pan, heat olive oil, add spinach and cook until wilted. Remove from heat.
Make Sauce: In a saucepan, melt butter over medium heat. Add flour and cook, stirring, for about 2 minutes. Gradually whisk in milk until smooth. Cook until thickened, then add salt, pepper, and nutmeg.
Combine Ingredients: Stir the spinach mixture into the sauce. Allow it to cool slightly. Beat the egg yolks and fold them into the spinach mixture.
Fold in Egg Whites: Gently fold in one-third of the beaten egg whites to lighten the mixture. Carefully fold in the remaining egg whites until no white streaks remain. Add cheese to mixture and gently combine.
Prepare Puff Pastry: Roll out the puff pastry sheet to fit the ramekins/baking dish. Place it in the dish, ensuring it covers the bottom and sides.
Assemble and Bake: Pour the spinach mixture onto the puff pastry. Bake in the preheated oven according to the puff pastry package instructions or until the pastry is golden brown.
Serve: Let it cool slightly before serving. The puff pastry will add a flaky and crispy layer to your spinach soufflé. Add Monterey Jack cheese to mixture and stir well to combine.