Preheat the oven to 180˚C/350˚F/Gas Mark 4
Cut the heads of garlic in half horizontally and drizzle with the olive oil. Wrap in foil and place on a baking sheet. Roast the garlic for 40 minutes, until completely soft.
Melt the butter in a large saucepan, and add the onions. Cover and cook over a gentle heat for 10 minutes, until the onion is soft and translucent. Add the frozen peas and stir so that the peas are coated in the butter and onion mixture.
Pour in the stock and bring to the boil. Turn down the heat and simmer for 5 minutes. Remove the garlic from the oven and, when cool enough to handle, squeeze the soft roasted garlic cloves into the pan.
Puree with a hand blender and season to taste with sea salt and black pepper. Serve the soup with a spoonful of crème fraiche, garnished with a few chopped chives.