Toss cantaloupe, onion, parsley, mint, vinegar and oil in a bowl. Set aside.
Heat a grill, grill pan or large skillet over high heat. Season flank steaks on both sides with salt and pepper. Cook 5 to 8 minutes on each side for medium rare, 8 to 10 minutes per side for medium and 10 to 12 minutes per side for well done.
Remove from heat and allow to rest on a clean cutting board for 5 to 7 minutes.
Cut steak diagonally into 1/8-inch-thick slices and transfer to a platter. Spoon half the relish on top.