Heat half the butter in a medium saucepan over medium heat and add the onion. Fry the onion for about 3 minutes, until fragrant and translucent. Add the spinach and peas and stir for about 5 minutes until the spinach has wilted. You can add a little of the stock during this process if the pan is too dry. 

Add the stock and reduce heat to a simmer. Season with salt. Simmer for about 5 minutes, then remove from the heat. Allow to cool for a minute or two, then transfer to a blender. Carefully blend the mixture (see Adam's tip) to a smooth puree.

Taste and adjust the seasoning if necessary. Divide the soup between bowls. Heat remaining butter in a small pan over medium heat until it turns medium brown and has a nutty aroma (about 4 minutes). Drizzle about a teaspoon over each bowl and add a grind of black pepper.

Adam's tip When mixing hot liquids in a blender, don't allow pressure to build up in the container. Cover it with a lid but be sure to leave the centre cap open. Start at a very slow speed and increase it gradually.