For the marinated tuna:

Combine the olive oil, herbs, fennel seed, and pepper to taste and spread over the tuna. Cover and refrigerate without turning for 8 hours or overnight.

When ready to cook, remove the tuna from the oil, leaving some seasonings on its surface. Sprinkle with salt and pepper. In a large skillet over high heat, sear the tuna on all sides, 2 to 3 minutes, until cooked rare.

For the vegetables:

Place the potatoes in a medium saucepan with cold salted water and bring to a boil over high heat. Reduce the heat to medium and simmer until tender, about 15 minutes. Drain and cut into quarters. While still warm, toss with ¼ cup of the sherry vinaigrette. Taste and re-season, if necessary.

Half-fill a medium saucepan with salted water, bring to a boil over high heat, and blanch the haricots verts until tender, 2 to 3 minutes. Immediately plunge into ice water and drain in a colander for 5 minutes. Toss the beans with the remaining sherry vinaigrette, and check the seasoning.

To serve:

Divide the potatoes and haricots verts among 4 serving plates, arranging them in the center. Slice the tuna ¼ inch thick and fan the slices around the vegetables. Drizzle the tuna with the roasted garlic-black olive oil. Garnish with the herbs, if desired.