Preheat the grill or grill pan over high heat. Whisk together the miso, oil, agave, water, and rice wine vinegar. Season to taste with salt. Rub the tuna steaks all over with the mixture and place on the grill. Evenly sprinkle 1 tablespoon of the sesame seeds on the fish. Grill on the first side for 3 minutes, flip, and sprinkle the cooked sides with the remaining sesame seeds. Grill for an additional 3 minutes and then remove to a plate. I like the tuna to be just cooked through, but feel free to grill for less time if you prefer it medium or even rare.
To serve, grill or toast the buns. Lay a few lettuce leaves on the bottom half of each bun. Cut the tuna into 1/2-inch slices across the grain and evenly distribute on the lettuce. Drizzle each sandwich with a tablespoon of the vinaigrette, being sure to get plenty of sliced shallots in each spoonful, and sprinkle with the cilantro leaves. Close each sandwich with the top halves of the buns and serve with lots of napkins.
Whisk the vinegar, agave, and soy sauce together. While whisking, drizzle in the vegetable oil. Add the hot pepper sesame oil to taste to stir it well before serving.
Reprinted with permission from My Father's Daughter: Delicious, Easy Recipes Celebrating Family & Togetherness by Gwyneth Paltrow, copyright (C) 2011.