The fish at San Diego's family-owned sandwich and taco stand Loaf & Fish varies by the week and seasons, from rockfish to black cod. This version of their sandwich uses the local Tuna Harbor Dockside Market’s namesake seafood.
Tuna bellies, or toro, are prized in sushi bars for their velvety texture. That said, you can also use tuna loin in this recipe; it will simply be a little lean. Make sure that your knife is very sharp—it’s the key to slicing through the soft fish easily without tearing it.
A whole meaty tuna loin can be roasted the same way as a hunk of beef. If your loins are small—3/4 lb. or less each—you can tie them together to create a plumper-looking roast: With the widest sides together, tie at 2-in. intervals with kitchen twine. Take the tuna out of the oven when it’s still very rare—albacore keeps on cooking after it’s pulled off the heat, and if it’s overcooked, it’s dry.