Step 1
Prepare the filling.
In a bowl, lightly beat the egg with a fork, add a pinch of salt and pepper. Add the chopped onion and spinach, and mix.
Step 2
Assemble the sandwich.
Lightly butter or oil the outer sides of both slices of bread — this is essential for a golden and crispy crust.
Pour the entire egg and spinach mixture onto the uncoated side of one slice of bread. Spread it out with a spatula or spoon, leaving about a 1 cm border around the edges to prevent the filling from leaking out during cooking.
Top with the second slice of bread, with its oiled/buttered side facing outward.
Step 3
Cooking in the Waffle Maker.
Preheat your waffle maker to a medium temperature (if it has an adjustable thermostat).
Carefully transfer the assembled sandwich to the center of the lower cooking plate.
Gently but firmly close the lid. The waffle maker will press the sandwich slightly.
Cook for 3-6 minutes (time depends on your appliance's power), until the bread is golden brown and crispy, and the egg inside is fully cooked.
Open the lid, carefully remove the sandwich with a spatula, and let it cool for about 1 minute before slicing.