Whisk the dressing ingredients together in a small bowl. Drizzle about a quarter of the dressing over the avocados and allow to soak as you prepare the tuna.  Brush both sides of the tuna with olive oil and sprinkle generously with salt and pepper.  Heat a skillet over high heat until the pan is smoking.  Add the tuna and cook for 1 minute on each side (maybe less). It will be seared outside and raw inside. Allow the tuna to cool slightly and cut into bite-size cubes. Place the sesame seeds in a dry skillet and cook over low heat, tossing often, until the seeds turn golden brown (about 5 minutes).  Combine the tuna, avocado, red onion and romaine in a large salad bowl.  Add dressing to taste and toss (about another quarter of the dressing recipe).  Arrange on individual plates, sprinkle with green onion and sesame seeds and serve at room temperature.