The tuna is sauteed only briefly, so ask your fishmonger to cut the steaks as uniformly as possible to ensure even cooking.
Photography: Anita Calero
Source: Martha Stewart Living, October 1996
1/4 cup finely chopped Italian parsley
3 1/2 tablespoons olive oil
2 tablespoons capers, finely chopped
4 anchovy fillets, finely chopped
2 teaspoons finely chopped fresh rosemary
Salt and freshly ground black pepper, to taste
4 half-inch-thick 5-ounce fresh tuna steaks
4 half-inch-thick slices peasant bread, toasted
2 bunches arugula (about 8 cups), stems trimmed to base of leaf
Grate zest and squeeze juice from lemons; transfer both to a shallow, non-reactive dish large enough to hold tuna steaks. Stir in parsley, 3 tablespoons oil, capers, anchovies, and rosemary. Season with pepper. Coat tuna in marinade; let stand 1 hour, turning steaks several times.
In a large cast-iron skillet, heat remaining 1/2 tablespoon oil over high heat. Transfer tuna to skillet, reserving marinade. Sprinkle with salt; cook until golden brown outside, cooked to desired doneness inside, about 1 minute per side for medium rare. Place tuna on toast; set aside.
Add arugula and 2 tablespoons reserved marinade to skillet; saute, tossing constantly, until wilted, 1 to 2 minutes. Divide arugula among sandwiches; top each with the tapenade.