Prepare the ingredients: Wash and dry the spinach and mushrooms. Slice the mushrooms if they aren't already sliced. Crumble the feta cheese or grate the cheddar cheese, if using.

In a medium bowl, whisk the eggs with salt and black pepper until well combined and slightly frothy.

Heat a non-stick skillet over medium heat and add 1 tablespoon of olive oil.

Add sliced mushrooms to the skillet and sauté for about 3 minutes, or until tender and slightly golden.

Add the fresh spinach to the mushrooms and sauté for another 2 minutes until the spinach is wilted. Remove the mixture from the skillet and set aside.

Pour the whisked eggs into the same skillet and cook over medium-low heat. As the eggs begin to set, gently run a spatula around the edges, tilting the pan to let the uncooked eggs flow to the edges.

When the eggs are mostly set but still slightly runny on top, add the spinach and mushroom mixture evenly over one half of the omelette. Sprinkle the feta or cheddar cheese over the top, if using.

Using a spatula, fold the omelette in half, covering the filling. Cook for another minute or until the cheese is melted and the eggs are fully set.

While the omelette is cooking, toast the whole-grain bread slice to your desired level of crispiness.

Serve the omelette with the toast on the side. Slice the orange into wedges and serve half alongside each plate as a refreshing complement.