If using, add rice, chicken or veggie stock, and coconut milk to the rice cooker and turn on. Otherwise, combine the three ingredients in a medium sauce pot. Bring to a boil, then reduce heat to medium-low and let simmer undisturbed until rice is tender, about 25 minutes.
Using a mandoline or sharp knife, thinly slice cucumbers and red pepper into rounds. Combine rice vinegar and sugar together and stir until completely dissolved. Pour vinegar into a quart sized zip-top bag with cucumber and pepper slices and seal, removing as much of the air as possible so veggies are coated in the solution. Set aside.
Meanwhile, pat tuna steaks dry with a paper towel. Brush with 2 Tbs sesame oil and lightly season with salt, bearing in mind you will be topping the steaks with fish sauce and lime juice which accentuate salty flavors. Generously pat ground pepper, coriander, and sesame seeds onto the steaks until the surface is completely covered. Set aside.
Combine chopped cilantro, lemongrass bulb medallions, scallions, fish sauce, and lime juice in a medium bowl and stir. Set aside.
Heat a cast iron skillet over high heat until smoking. Add roughly 3 Tbs of sesame oil to the pan, followed by your steaks. Steaks are cooked after 2 minutes per side, but I prefer my steaks closer to 4 minutes per side. Transfer to a cutting board and let cool. Cut into thin strips and place on a plate with coconut rice and drained quick pickles. Top steak strips with cilantro mixture. Serve immediately.