Melt butter in medium saucepan over a medium heat and sauté the onions until golden brown.
Pour in the cream, the Worcestershire sauce and the mustard. Give it a good stir and bring up to a gentle simmer.
Crumble in the Puhoi Valley Distinction Blue Cheese and stir until fully melted.
Serve the sauce with steak, poured over baked potatoes or tossed through fresh pasta.