Step 1

To begin, make the crema. Place the double cream in a pan and heat until it reaches 35°C. Remove from the heat, transfer to a glass jar and pour in the buttermilk

Step 2

Loosely place the lid on the jar and set aside in a warm place for 24 hours. After this time, tighten up the lid and store in the fridge

Step 3

Trim and clean the spring onions and preheat a griddle pan or better still, a barbecue. If you're using a pan, wipe the surface with some neutral oil (like rapeseed oil) and place the spring onions over a medium-high heat

Step 4

Cook until the spring onions are nicely blackened, turning occasionally until deeply charred and caramelised

Step 5

When the onions are ready, transfer to a high-powered blender and blitz with the crema (or sour cream) until as smooth as possible. Season to taste