Place halibut bones in a medium saucepan and add just enough water to cover. Simmer on low heat 20–25 minutes.
Rinse the kombu and set in a large pot or heat-resistant container. Place a strainer or sieve over the top of the container. Once the fishbone stock is ready, pour it over the strainer directly onto the kombu. Cover with a lid or plastic wrap and let steep 15 minutes. Strain the resulting dashi through a fine strainer or cheesecloth into a large bowl. Cool the dashi and season with a combination of white soy sauce and kosher salt until it tastes good to you.
Using a very sharp knife, preferably a slicing knife, make long slices perpendicular to the grain of the muscles in the fish. The lines that you see are the direction of the grain—you are slicing across them, not with them. Make long, careful strokes using the whole length of the blade, by pulling back with your elbow and letting the knife do the work; do not push down with the knife or “chop” the fish. Slices should be as thin as you can get, about 2 millimeters for a firm-fleshed fish. Depending on the size of the loin, you are looking for 6 to 9 slices or 3 ounces.
Lay the slices of fish on a rimmed plate. Pinch a few flakes of Maldon salt from high over the fish, then top with the seasoned dashi. Splash a healthy glug of arbequina olive oil, and top with the toasted white sesame and sliced green onion. Gently shake the plate to lightly mix the dashi and oil, and serve with warm bread.