Preheat oven to 180 C / Gas 4. Grease a medium-sized oven proof baking dish and set aside.
Melt the butter in a medium pan over medium-low heat. Add flour and mix with a wooden spoon to make a roux. Pour in the milk and stir constantly with a whisk until obtaining a smooth and thick bechamel. Remove from heat and add drained spinach and season with salt, pepper and nutmeg. Add 2 tablespoons of mayonnaise, half of the grated cheese, and mix well.
Heat 1 tablespoon of olive oil over medium heat. Cook and stir the chopped leek until slightly tender. Remove from heat and spread evenly over the previously greased baking dish.
Season the flounder fillets with salt and pepper and set over leek. Pour sauce over fillets and generously sprinkle with the remaining grated cheese.
Bake in the preheated oven for 35 to 40 minutes, or until fillets are fully cooked and easily flake with a fork. Remove from the oven and serve.