Cut pretzel sticks into slices and place in a suitable bowl.
Quarter apple, core and cut into cubes. Cut leek into slices and soak in standing water. Drain and sauté with apples in butter for 2 min, season with salt and pepper.
Mix eggs and milk, add pepper, salt, nutmeg and mix everything together. Grate Gouda on a box grater and stir in.
Pour into a buttered gratin dish and bake in a heated oven at 200 degrees for 35 to 40 minutes on the 2nd rack from the bottom.