Start by combining the ingredients for the teriyaki sauce (except the cornstarch slurry) in a small saucepan and set aside. Next, prepare your salmon. Sometimes, salmon fillets will have scales still on them. I like to use a serrated knife to carefully scrape them off. Then rinse and pat the salmon dry with a paper towel.
Rub the salmon with salt, pepper, and garlic powder. If using wasabi, you can take a small dab of wasabi and rub it onto each piece of salmon now (you can also just serve it on the side later).
Preheat your oven to 400 degrees F. Take the saucepan you prepared earlier and bring the sauce mixture to a simmer. Stir in the cornstarch slurry, and continue to simmer for 5 minutes, or until the sauce is thick enough to coat a spoon. Turn off the heat.
Add a tablespoon of oil to a cast iron skillet or other thick, oven-proof pan, and place over medium heat. Place the salmon in the pan skin-side down, and brush with a small amount of teriyaki sauce. Be careful not to let the sauce drip down the sides to the pan; if it does get to the pan, it will burn and become bitter. Sear the salmon for 3 minutes.
Brush a bit more teriyaki sauce onto the salmon, again being careful to use just enough so that it doesn’t drip down to the pan, and transfer to the preheated oven. After 3 minutes, brush more sauce on the salmon and then again after another 3 minutes.Use a knife to poke the thickest part of the salmon and spread the meat to take a peek inside. Take the salmon out of the oven early if necessary because there is a world of difference between perfectly cooked and overdone salmon!
After a final 2 minutes of baking, remove the salmon from the oven. (The salmon will have been roasting for a total of 8 minutes). Note: If you like your salmon medium rare, you can reduce the cooking time by 2 or 3 minutes.
Serve the salmon over rice and drizzle with more teriyaki sauce. Sprinkle with toasted sesame seeds and roasted seaweed. Enjoy immediately with more teriyaki sauce and wasabi on the side!