Meanwhile, prepare the grill, adjusting the rack 2 inches above the coals. Trim the tuna, discarding the skin and any dark purple flesh. Refrigerate.
03Cut the roasted peppers lengthwise into 1/4-inch-wide strips. Put in a bowl and toss with 1 tablespoon of the olive oil. Season with salt and pepper and set aside.04Drain the potatoes and peel while still hot. Slice into 1/4-inch rounds and arrange 1/4 of them in a pie plate or casserole. Sprinkle with 1/4 of the shallots, 1/4 of the parsley and salt and pepper. Repeat the layers until all the potatoes are used. Whisk 2 teaspoons of the vinegar with 1 tablespoon of the oil and pour over the top.05Lightly rub the tuna steaks with 1 tablespoon of the olive oil and season with salt and pepper. Grill until charred on the outside and rare in the center, about 2 minutes per side.06Place the greens in a large salad bowl and top with the potato mixture, the peppers and the tomatoes. Slice the tuna into 1/4-inch strips and arrange on top of the salad. Just before serving, make a vinaigrette from the remaining vinegar and oil and pour over the salad. Season with salt and pepper. Taste and toss with more vinegar or oil if needed. Serve immediately.