Melt the butter in a saucepan and sweat the garlic and onion until soft. Add the stock, bring to the boil, add the peas, bring back to the boil, and simmer for 45 seconds. Remove from the heat. Strain the peas reserving the cooking liquid. Place the pea mixture in a blender and blitz to a puree consistency, adding reserved liquid if need be. When smooth, chuck the butter in too, then strain through a sieve. Season to taste and reserve in a piping bag in the fridge. It should be quite firm when cold.