In a large saucepan, melt the butter in the olive oil. Add the leek; cook over moderately high heat until tender, about 1 minute. Add the stock, cover and bring to a simmer. Add the sugar snaps and cook over moderate heat until just tender, about 2 minutes. Stir in the pasta, spinach, lemon juice and lemon zest; season with salt and pepper. Cook just until the spinach is wilted, about 2 minutes. Ladle into bowls and serve.