Sashimi generally refers to any thinly sliced, raw saltwater fish, but can include other types of seafood. You should only marinate sashimi briefly, however, no matter what the type. Most marinades, especially acidic ones, toughen the surface of sashimi after 30 minutes and muddle its fresh, clean flavor. However, vinegar with at least five percent acetic acid does a lot to inhibit bacterial growth, which is one of the benefits of sushi rice. Two main types of marinades work best with sashimi: traditional magurotake, a marinade that focuses on umami, or the fifth taste, and oil-based marinades, which are usually what you see in Western cooking.
Whisk together two parts soy sauce to one part sake in a mixing bowl for the base. Add a splash or two of rice wine vinegar to the marinade and whisk. The rice wine vinegar is for flavor and preservation.
Add a few splashes of mirin, sweet sake or shiro dashi to the marinade and whisk. The sugar in the mirin or sweet sake complements the acid in the soy sauce.
Add aromatic ingredients to the marinade to taste. This is where you can get creative and mix Western and Eastern flavors, or keep it traditional. A traditional magurozuke might include a combination of sliced leeks scallions and ginger, wasabi paste, bonito flakes and shichimi togarashi, or Japanese seven spice. Other options include minced garlic, shallots, chili flakes and black peppercorns.
Whisk the marinade once more and place the sashimi in a sealable food storage bag. Pour enough marinade in to cover the sashimi by about 1/4 inch when it lays flat. Shake the bag to coat the sashimi and place it flat on a plate on the bottom shelf of the refrigerator.
Marinate the sashimi for up to 30 minutes, turning the bag over after 15 minutes. Remove the sashimi-grade seafood from the bag and wipe the marinade from it before slicing.
Whisk together two parts flavored oil to one part acidic liquid in a mixing bowl. Two of the most common flavored oils include grapeseed and extra-virgin olive oil, but you can also use coconut, palm or sesame oils. For the acidic component, you might use soy sauce, Worcestershire sauce or freshly squeezed lemon juice. The acidic liquid is for flavor, not tenderness, so use what tastes best to you.
Add a few splashes of flavored vinegar with five-percent acetic acid to the marinade. The vinegar is for both preservation and flavoring.
Add the herbs and spices of your choice to the marinade. Again, you can go as bold or as subtle as you want with the spices. Cilantro, coriander, parsley, lemongrass and rosemary are some balanced, subtle flavors that go with sashimi. Other options include peppercorns, lemon zest, dill, shaved fennel and capers.
Season the marinade to taste, then whisk.
Place the fish in a sealable food storage bag and pour enough marinade in to cover it by 1/4 inch when it lays flat. Place the bag on a plate on the bottom shelf of the refrigerator.
Marinate the sashimi for 30 minutes, turning it over at the halfway point. Remove the sashimi from the marinade and wipe it off with paper towels before slicing.