Chop spinach and mushrooms into bite-sized pieces.

Mix eggs and milk thoroughly in a bowl.

Combine spinach with the egg mixture.

Heat half the butter in a pan and sauté mushrooms with ground black pepper.

Remove sautéed mushrooms from the pan and set aside.

Add the remaining butter to the pan and melt it.

Pour the egg mixture into the pan and cook on low heat.

Stop cooking when the center of the omelette is slightly runny.

Place a slice of cheese on top of the omelette.

Add the sautéed mushrooms on top of the cheese.

Fold the omelette in half and cook briefly to set the shape.

Transfer the folded omelette onto a plate.