Melt 30g of the butter and 1 tablespoon oil in a large frying pan over high heat until foaming. Add the mushrooms and cook, tossing often for 4-5 minutes or until light golden and tender. Add the Worcestershire sauce and toss to combine. Keep warm.
Beat two eggs, 1 tablespoon milk and 1 tablespoon chives and pepper to taste, in a bowl until combined.
Add a little remaining butter and oil to a non-stickmedium frying pan and heat over medium heat. Pour in the egg mixture and swirl to coat the base. Cook uncovered for 2-3 minutes until golden underneath. Top with one-quarter of the chives, cheese and mushrooms, cover and cook 1-2 minutes until cheese is melted. Fold the omelette over to cover mushrooms, slide onto a plate. Keep warm.
Repeat with remaining ingredients to make three more omelettes. Serve with buttered toast.