Peel mangoes and dice the flesh into 1/4-inch pieces. Transfer to a small bowl. Add chopped cilantro and a pinch of salt and mix well. Place mango relish in the refrigerator to chill for at least 30 minutes.
Lightly press coarsely ground black pepper into one side of each tuna steak. Sprinkle with salt. Place the fish, pepper-side down, in a large skillet coated with nonstick cooking spray. Cook on both sides over medium-high heat until the fish is seared on the outside but still pink in the center. Cut seared tuna on the bias into 1/2-inch-thick slices.
Mound mango relish in the center of each plate. Arrange tuna slices so they overlap and form a circle around the relish. Garnish with whole fresh cilantro leaves.