Put the tuna steaks between two sheets of plastic wrap and pound to a ½ inch thickness with a heavy pan or the flat side of a meat pounder. Put the tuna steaks in a Ziploc plastic bag with the 2/3 cup of olive oil and the rosemary and marinate in the refrigerator for 4 to 6 hours.
Trim the stems and about 1 inch of the top of the fennel bulbs. Thinly shave the fennel into 1/8-inch thick half-moon shapes. Discard the core, dark green parts, and tough root end or outer layers of the bulb. You should end up with about 2 cups of fennel strips.
Segment the oranges into supremes, working over a bowl to catch the juice.
To segment the oranges, slice off about 1/2-inch from the top and bottom of the oranges. Working with one orange at a time, stand an orange on end and cut downward all around, cutting just beneath the white pith down to the flesh to remove the entire rind in strips. Steady the orange and remove one segment at a time by running the knife in a V shape as close to each side of each membrane as possible, releasing each segment into a bowl. Squeeze the remaining accordion-like "skeleton" of membrane to extract the juice.
Measure out 1 tablespoon of the orange juice, pour it into the bowl and discard the rest. Gradually whisk 3 tablespoons olive oil into the juice in a slow, steady stream until blended. Taste and season lightly with salt and pepper. Add the fennel, parsley and oranges, taste, and season again with salt and pepper, tossing to coat.
Coat a pan with a 1-2 Tbsp olive oil, depending on the size of the pan, and heat over medium-high heat. Lightly season the tuna steaks with salt and pepper and let stand at room temperature until the pan heats up. When the pan is hot, grill the tuna steaks on one side only, 1 to 2 minutes. Use a wide metal spatula to flip the tuna steaks onto plates so that the grilled side is up.
Arrange the salad over the tuna steaks, drizzling some of the vinaigrette on top. Squeeze fresh lemon juice over the dish and serve. Makes 4 servings.
Recipe and image used with permission and courtesy of Wusthof. All rights reserved. Recipe by Marc Vetri, Chef and Owner of Alla Spina, Amis, Osteria, Vetri, Philadelphia, PA.