The ahi tuna burger is decided by how far you stop short of cooking it through. A ground-beef burger is built around a hard sear and a fully cooked interior; the ahi version keeps the bun, the sear, and the architecture, then deliberately leaves the center raw. A thick ahi steak is seared hot and fast on two faces so a thin crust forms while the middle stays cool and translucent. That contrast between a browned skin and a raw rose interior is the entire sandwich, and it is the opposite of what a beef patty is asked to do.
The craft is timing measured in seconds rather than minutes. Ahi is lean and firm, and it has no fat to render and no connective tissue to break down, so heat held too long turns it dry and chalky in the way an overcooked beef patty never quite does. The fillet is left as a single steak rather than ground, because grinding would destroy the only texture the fish offers and there is no fat to bind it back together. The sear is driven on a flat-top or a grill at high heat to build flavor on the surface before the heat reaches the core; the moment it does, the sandwich is over. The bun is the same engineering problem as on a beef burger but with a leaner cargo: a soft, slightly sweet bun that compresses to the fish, often toasted on the cut face so a thin seared layer slows any moisture from a slaw or a sauce reaching the outer shell. The standard counter is built for a clean protein, not a fatty one: a wasabi or ginger mayonnaise for heat and richness, pickled vegetables or a crisp slaw for acid and crunch, since the fish brings neither.
The variations stay inside the seared-rare frame. A sesame-crusted build presses seeds into the surface so they toast into the crust. A blackened version trades the seed crust for a spice rub driven hard on the flat-top. A fully grilled, cooked-through reading abandons the rare center and becomes a different, firmer sandwich closer to a plain fish fillet on a bun. Each of those is its own preparation and deserves its own article rather than being crowded in here.