How do you make the seafood sauce at Japanese Steakhouse restaurants?

You don't. They already have it made for you.How do you make the seafood sauce at Japanese Steakhouse restaurants?
Based on the limited information given. It's probably a combination of Dashi and soy sauce. Dashi is a Kombu (kelp) based broth. You can definitely buy it in your local Japanese market. If you have a large enough Japanese population you might be able to buy it in the grocery store (that's where we get ours).