Combine the marinade ingredients in a glass or ceramic bowl. Place the tuna in the marinade, and allow it to marinate in the refrigerator for about 30 minutes. Prepare a charcoal fire for grilling, or preheat the broiler.
Peel four of the tangerines, and pull them apart into sections. Put them into a bowl. Cut and squeeze the remaining two tangerines, and pour their juice over the sectioned tangerines.
Grill or broil the marinated fish until it is done to your liking (it should be cooked through and springy to the touch). Cut each piece of tuna across the grain into 3 to 4 slices. Drain the tangerine sections, reserving the juice. Arrange the lettuce on individual plates, top with the tuna slices and tangerine sections, and cover and chill the plates.
In a small saucepan, reduce the tangerine juice by one-fourth. Let the juice cool.
To make the vinaigrette, combine the vinegars, tangerine juice, and mustard in a bowl. Whisk in the oil, and season with the tangerine zest, fennel seeds, and salt and pepper. Just before serving, drizzle the dressing over the salad plates, and sprinkle with minced cilantro.