Whisk together the vinaigrette, lemon zest, garlic, and oregano in a medium bowl. Arrange the tuna steaks in a shallow baking dish and pour 1/2 cup of the mixture over them. Turn the tuna to coat, then cover and refrigerate for 1 hour. Heat a grill to high or a grill pan to medium-high. Grill about 4 minutes per side for medium-rare, depending on the heat level. Meanwhile, pour the remaining 1/4 cup of vinaigrette over the tomatoes, capers, and arugula and toss. Serve the tomato salsa over or alongside the tuna.
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