Melt 1/2 tablespoon of the butter in a medium saucepan. Add the onion and cook over moderate heat, stirring occasionally, until softened but not browned, about 3 minutes. Add the peas and stock and season with salt and pepper. Simmer over low heat for 15 minutes. Transfer the pea mixture to a food processor and puree until smooth. Return the puree to the pan and beat in the remaining 2 tablespoons of butter and the mint.