Peel the pears vertically with a vegetable peeler, trimming away the stem. Halve each pear, then core and cut into 8 wedges. Rub with lemon to keep from discoloring.
Heat the butter in a 12-inch sauté pan over medium-high heat. Add the pears in a single layer and sprinkle lightly with sugar. Cook over high heat for about 3 minutes. Brown lightly on both sides, turning with care so pieces do not break.
Fan out 4 pear slices on each plate and sprinkle lightly with black pepper. Drape 2 prosciutto slices over each plate and garnish with a sprig of mint.