Measure 2 tablespoons oil from the tin of anchovies and put it in a blender. Chop 3 anchovy fillets coarsely and add them to the blender. (Cover and store the remaining anchovies in the refrigerator for another use.) Add clam juice, vinegar, garlic, rosemary and pepper to the blender and blend until smooth.

Pour 3 tablespoons of the marinade into a shallow dish just large enough to hold the tuna. Add tuna, turning to coat. Cover with plastic wrap and refrigerate for 30 minutes. Season the remaining marinade with additional pepper; cover and refrigerate.

Meanwhile, cook potatoes in a large pot of boiling salted water until just tender, about 6 minutes. Remove the potatoes with a slotted spoon and set aside. Add green beans to the simmering water and cook until barely tender, 3 to 5 minutes. Refresh under cold running water and reserve.

Preheat grill to medium-high. Remove the tuna from the marinade. Add the onion pieces and cooked potatoes to the marinade left in the dish, turning to coat. Thread the potatoes and onions onto skewers; thread tuna onto separate skewers. Grill the vegetables until they are slightly blackened, 10 to 12 minutes, and grill the tuna until it is no longer opaque in the center, about 8 minutes.

Toss mache (or mixed salad greens) with 1/4 cup of the reserved dressing and transfer to a large platter. Toss the green beans with 2 more tablespoons of the dressing and scatter over the lettuce. Slide the tuna, potatoes and onions from the skewers onto the salad. Garnish with tomato wedges, olives, egg slices and capers. Drizzle with the remaining dressing.