Add 1 teaspoon of salt to the potato water and bring to a boil. Add the peas and cook over moderately high heat until tender, about 5 minutes. Meanwhile, in a large skillet, melt the butter in the olive oil. Add the leek and cook over low heat, stirring occasionally, until tender, about 5 minutes. Reserve 1/2 cup of the potato cooking water, then drain the peas and add them to the skillet.