Crack the eggs into a mixing bowl. Add a pinch of salt and pepper if desired. Whisk until the yolks and whites are fully combined and the mixture is slightly frothy.

Add the olive oil (or butter) to a non-stick skillet and heat over medium-low heat. When the oil is hot or the butter is melted and starting to bubble slightly, reduce heat to low.

Pour the eggs into the skillet, spreading them evenly across the surface. Let them cook undisturbed for about 1-2 minutes until the edges start to set.

Evenly distribute the sautéed spinach and crumbled feta cheese over one half of the omelette.

Using a spatula, gently fold the empty half of the omelette over the side with the spinach and feta. Cook for another minute until the cheese starts to soften but the omelette is still slightly soft and fluffy inside.

Remove the omelette from the skillet and transfer to a plate.

In the same skillet, toast the slice of whole-grain bread until golden brown on both sides. Alternatively, you can use a toaster if preferred.

Arrange the sliced tomato on the plate next to the omelette and toast. Optionally, sprinkle a little salt and pepper over the tomatoes for enhanced flavor.

Serve the omelette warm alongside the whole-grain toast and sliced tomato.