Tip: Make the slaw and dressing several hours ahead of time and chill separately, covered.
Using a sharp knife, trim any sinew from fish. Cut into 1/4-inch cubes and put in a medium bowl. Add cilantro, ginger, scallions, sesame oil, olive oil, garlic and salt; use your hands to mix gently but thoroughly. Cover and refrigerate mixture until very cold, about 30 minutes.With slightly moistened hands, form tuna mixture into 4 3/4-inch-thick patties. Place on a plate, cover and refrigerate until firm, about 15 minutes. Spread sesame seeds on a rimmed baking sheet or plate and coat each burger on both sides with seeds, pressing gently to help them adhere.Heat a large cast-iron or nonstick skillet over medium-high heat; add a thin film of canola oil to skillet. When oil is hot, add burgers and cook until sesame seeds are golden, about 2 minutes. Gently flip burgers and cook opposite side until seeds are golden, about 2 minutes longer. (Burgers will still be very rare in the center.)Transfer burgers to plates and serve with lettuce leaves. Serve with Spicy Slaw and/or Avocado–Hearts of Palm Salad.