Cut each tomato into four wedges, cut out the cores, and remove the seeds and pulp. Slice the tomatoes lengthwise into 1/4-inch strips. In a colander, toss them with 1/4 tsp. salt and let them drain for 15 minutes.
Meanwhile, finely grate the zest of the lemon. Put the zest in a medium bowl. Working over the bowl, segment the lemon and the oranges. In a spice grinder or mortar and pestle (or with a meat mallet; put the spices in a zip-top bag), coarsely grind the coriander and peppercorns. Press the spices into both sides of the tuna steaks.
Gently heat 3 Tbs. of the oil in a 12-inch skillet over medium heat. Add the anchovies and mash them into the oil with the back of a spoon until nearly dissolved. Turn the heat to low, add the garlic, and cook until softened but not browned, 3 to 4 minutes. Remove from the heat. Add the drained tomato strips, the orange and lemon segments (with the zest and juice), and the olives to the pan. Toss very gently to warm through, being careful not to break up the citrus segments. Season to taste with salt and pepper. Transfer to a serving bowl and keep warm.
Wipe out the skillet, set it over medium-high heat, and pour in the remaining 2 Tbs. oil. Generously salt the tuna steaks on both sides. Working in batches if necessary, sear the steaks, pressing on them while cooking to help a crust develop, until golden brown, 2 to 3 minutes. Flip the tuna and continue to cook until golden brown, another 2 to 3 minutes for medium rare to medium. Transfer the tuna to dinner plates and serve with the warm citrus sauce.