Measure out about 6 packed cups of the cabbage (or more, if you prefer) and place in a large mixing bowl and set aside.
Place all of the ingredients for the dressing into a bowl or food processor and mix until creamy. Taste and adjust, if necessary.
Place the cabbage and scallions in a large mixing bowl and drizzle with dressing. Toss to combine everything together and set aside.
Place the sesame seeds in a shallow dish or plate and toss to mix together. Set aside.
Place a large skillet over medium heat and add in the olive oil.
Meanwhile, press one side of the tuna steak down into the sesame seed mixture and then turn over and repeat.
Once oil is heated, add in the tuna steak, sesame seed side down. Sear for 1-2 minutes (depending on how well done you’d like it) and then flip over and cook another 1 minute. Remove the tuna and place on a cutting board. Slice thinly.
Divide the cabbage onto plates and top with the tuna steak slices.