Cut the fennel into very thin slices, spread it out on a flat surfaced utensil and add enough lemon juice to cover it. Cover the pan with cooking membrane and let the fennel marinate for at least 2 hours.
Pad the tuna with a towel so that there is absolutely no humidity in it, and carefully season with salt and pepper, as this is a fish with a strong taste and do not overdo it with salt.
Place the skillet over high heat, add some sunflower oil and sizzle the tuna for about half a minute on each side. Remove from heat, let it rest for 2-3 minutes, and then cut into thin slices.
Combine soy sauce, lime juice, rice vinegar, mirin, and brown sugar to make the dressing.
Place the pickled fennel on a long, narrow plate, top with the tuna slices and drizzle with the dressing. Decorate with green onions and brick, as in the photo.