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In a large, stainless steel mixing bowl, combine the ground tuna, chopped onions, cornmeal or polenta, egg, salt, pepper, and oregano. Let the mixture stand, refrigerated, for 30 minutes.
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Shape into 6 three-inch / 7.5-cm burgers.
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Heat 2 tablespoons olive oil in a ribbed, nonstick skillet. Place the tuna burgers a few at a time in the hot pan and sear, turning once, until browned on both sides. Cook to desired doneness.
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To make the relish: mix together the yogurt, olive paste, lemon juice and pepper in a small bowl.
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Serve the burgers hot, over pumpernickel, whole wheat, or olive bread, topped or garnished with the tomato slices, onions, and arugula, and with the yogurt-olive relish on the side.