Bring broth to a simmer in a medium saucepan (do not boil). Keep warm over low heat.
Melt butter in a medium sauté pan over medium heat. Add leek and garlic; cook for 3 minutes, stirring frequently. Add rice and 1/4 teaspoon salt; cook 1 minute, stirring constantly. Stir in wine; cook 2 minutes or until liquid is nearly absorbed, stirring constantly. Add broth, 1/2 cup at a time, stirring frequently until each portion of broth is absorbed before adding the next (about 20 minutes total). Stir in chopped sage, and cook for 2 minutes. Remove from heat; stir in mozzarella. Spoon 1 cup risotto into each of 4 bowls; top each serving with about 1 1/2 tablespoons prosciutto. Sprinkle with black pepper. Garnish with sage sprigs, if desired.