In a pot, melt the butter over a low to medium heat. Add in the diced onion, and cook for 3-5 minutes or until the onion has turned translucent and is cooked through.
Add the Tatua Crème Fraîche and mustard to the onion mixture. Cook while stirring for 1-2 minutes, then add the cheese and cook for a further 1-2 minutes or until the cheese has melted. Season to taste and serve over pasta, meat or your favourite vegetables.
