1. Using a vegetable peeler, remove half of cucumber peels in alternate strips. Halve cucumbers lengthwise; scoop out and discard seeds, then cut cucumbers into a 1/4-inch dice. In a small bowl, combine cucumbers, olives, and cilantro. Add lemon juice and 1/4 cup oil; season with salt and pepper.

2. Season tuna with salt and pepper. Heat a large, heavy skillet (preferably cast-iron) over high. Add 2 tablespoons oil; when it begins to shimmer, add 3 tuna steaks. Sear 1 minute, then flip and cook 30 seconds more. Add 2 tablespoons butter, swirl to melt, and cook 10 seconds more. Transfer fish to a platter, tenting with foil to keep warm. Wipe out skillet and repeat with remaining 1 tablespoon oil, 3 tuna steaks, and 2 tablespoons butter. Serve tuna topped with relish, with lemon wedges and more relish alongside.