For the blanched spinach with sesame dressing: Place sesame seeds in a grinding bowl (mortar and pestle) and grind fine. Transfer to a small mixing bowl; add sugar, soy sauce, and sesame oil and whisk to blend well. Steam or boil the spinach for 1 minute (until wilted), and then squeeze to drain well. Place the drained spinach in a medium-size mixing bowl and toss with the dressing to coat well.
For the tuna: Place a skillet over medium-high heat and coat with the olive oil. Season the tuna generously with salt and pepper. Lay the tuna in the hot oil and sear for 1 minute on all 4 sides to form a slight crust. Remove tuna from heat. Use a very sharp knife and slice the tuna 1/4-inch thick until all has been sliced. Serve the seared tuna slices with the sliced avocado. In a small bowl add vinegar, soy sauce, cilantro and then whisk in the olive oil to blend. Drizzle over the tuna.