Be sure to grab a loaf of crusty bread to serve with the mussels. Silky, savory broth like this is made for sopping.
Heat butter and olive oil in a large stockpot over medium-high heat. Add leek to pan; cook 1 to 2 minutes or until softened, stirring occasionally. Add wine, lemon rind, and juice to pan; bring to a simmer, and cook 2 minutes.
Add mussels to pan. Cover; cook 3 minutes or until mussels open, gently shaking pan 1 to 2 times during cooking. Remove pan from heat.
Spoon mussels into a large shallow serving bowl. Stir salt and pepper into broth, and pour over mussels in bowl. Serve immediately.