Cut cantaloupes into matchsticks. Lay a few cantaloupe sticks on the end of a prosciutto slice, crosswise. Roll into a log, then cut in half.
Melt butter in a non-stick pan. Add a handful of sage leaves. Cook until just crispy. Sprinkle crostini with grated fontina cheese. Broil in oven until cheese has melted, 2 min. Top with prosciutto and sage leaf. Serve warm.
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