Heat the butter in a large saucepan, add the leeks and onion, and cook over medium-low heat for 5 to 10 minutes, until the onion is tender. Add the chicken stock, increase the heat to high, and bring to a boil. Add the peas and cook for 3 to 5 minutes, until the peas are tender. (Frozen peas will take only 3 minutes.) Off the heat, add the mint, salt, and pepper.