Melt butter in a large saucepan over medium heat. Add leek, thyme, and tarragon to pan; cook 4 minutes or until leek is tender, stirring frequently. Add broth and cream; bring to a simmer. Add peas and mussels; stir well. Reduce heat to medium-low; cover and cook 7 minutes or until mussels open. Remove from heat; discard any unopened shells. Sprinkle evenly with mint and pepper.