In a large, heavy-bottomed saucepan (preferably untreated cast iron or stainless steel), melt the butter over medium-low heat. Once the butter is melted and beginning to foam, add the sliced onions, a generous pinch of salt, and cover the pot with a lid.

Uncovering to stir occasionally to prevent browning, cook the onions until they have softened and decreased in size by about half - about 20 minutes.

Uncover the pot and, stirring frequently, cook the onions until they take on a uniform browned colour and have softened completely. If you notice the onions browning or burning around the edges, add a splash of water to the pot. Once the onions have reached a uniform deep amber colour, remove from the pot and allow to cool slightly.

In a large mixing bowl, whisk together the ricotta, yoghurt, chives and Worcestershire sauce. Season with salt and pepper to taste.

Fold in the caramelised onions and taste to adjust for seasoning. Transfer to an airtight container and refrigerate for at least one hour before serving.