Soak the hijiki seaweed in cold water for 20 minutes. Drain and set aside. In a saute pan, heat the oil. Slit the skin on the top of the salmon to prevent the salmon from buckling. Season the salmon with salt and white pepper. When the pan is smoking hot, saute the salmon for 1 1/2 minutes. Turn and continue cooking for 35 seconds. Remove from the pan. In the same pan, melt 1 tablespoon of butter, add the leeks, and then reduce the heat. Simmer the leeks for 2 minutes. Add the preserved ginger and deglaze with rice vinegar. Add the mirin and fish stock, bring to a boil, and remove from the heat. Stir in the hijiki seaweed and the oysters, and allow the oysters to warm through. Add the shiso, whisk in the remaining butter and season with salt and white pepper. To assemble, spoon the oysters and leeks evenly into a shallow bowl. Place the salmon on top and spoon in the broth.