Add wine, lemon slices, and 1/4 cup parsley; bring to boil. Add mussels and tomatoes. Cover pot and cook until mussel shells open, stirring once, about 6 minutes; discard any mussels that do not open. Using slotted spoon, transfer mussels to large shallow bowl. Boil broth in pot until reduced to 1 cup, about 3 minutes; season, to taste, with pepper. Pour broth over mussels. Sprinkle with remaining parsley and fronds.