In a small bowl, whisk together the miso, mirin, lemon juice, mustard, Sriracha, and sesame oil. Add the tuna, avocado, and scallion and toss to combine. Store refrigerated, until ready to serve, up to 4 hours.
Spoon the tuna poke into the endive leaves. Sprinkle with toasted sesame seeds, garnish with the nori, and serve