In a separate, large, non-stick frying pan melt 1 tablespoon of butter on a low heat, taking care not to burn it. Pour in the eggs and cook for about 2 minutes.
Cook' note: the slower you cook it, the better the omelette.
When the omelette is cooked around the edges but still runny in the middle, add mushrooms. Cook until the eggs are firm, or to taste.
Using a spatula ease the omelette onto a plate and flip it in half. Slice into six portions.
Garnish with finely chopped chives and serve with a green salad.