Add the butter to a big, non-stick pan over medium heat.
Once melted, pour in the egg mixture.
Arrange the 4 gouda slices and the baby spinach on top.
Cover with a lid and turn the heat to low.
After 1 minute, remove the lid and, using a spatula, fold the omelette, rolling it into 4.
Turn off the heat and serve.
Notes
Best served fresh and hot. For a fully vegetarian dish, use a rennet-free cheese. You can swap the spinach for other leafy greens or add sauteed mushrooms.