I usually use an Asian inspired sauce as mentioned above (ponzu-esque) but for a change will sometime make the mustard/mayo based dipping sauce used at Joe's Stone Crab Restaurant in Miami, FL. for thier stone crab claws.
Recipe is everywhere online and on thier restaurant site.
Dijon, Mayo, worchetshire sauce and a few other ingredients. I find it a nice break.
Here are a few dipping sauces I've used with tuna:
Mirin-Soy Dipping Sauce:
1 cup sugar
1/2 cup sake
1/2 cup soy sauce
1 1/2 teaspoons thinly sliced grilled green onions
3/4 teaspoon minced garlic
3/4 teaspoon minced ginger root
In a small saucepan, combine all the ingredients and bring to a boil. Cook until the mixture is reduced to about 1 cup with syrup-like consistency. Let cool before serving.
Yield: about 1 cup
Wasabi Soy-Honey Dipping Sauce:
1/2 cup soy sauce
1/3 cup rice wine vinegar
1 tablespoon wasabi powder
1/3 cup honey
1 tablespoon toasted sesame oil
In a medium bowl, combine the soy sauce, vinegar, wasabi, honey, and sesame oil. Stir to blend.
Sesame Dipping Sauce :
3 tablespoons sesame paste
1/2 cup shoyu (Japanese soy sauce)
2 tablespoons mirin
1 teaspoon red pepper flakes
Put the sesame paste into a bowl and slowly stir in the shoyu, mirin and red pepper flakes.
SHERRY-SOY WASABI MARINADE
1/2 cup canola, vegetable, or peanut oil
1/2 cup dry (not cooking) sherry
1/4 cup low-sodium soy sauce
2 tablespoons Asian sesame oil
2 tablespoons peeled and finely chopped fresh ginger
2 cloves garlic, peeled and finely chopped
1/4 cup coarsely chopped cilantro leaves, tightly packed
3 whole scallions, sliced
3 tablespoons rice wine vinegar (or white vinegar)
1 tablespoon Annatoo (Achiote) paste
2 tablespoons ground cumin
2-3 teaspoons Wasabi paste
Combine all ingredients in a saucepan and heat to just below the boil. Remove from the heat. Cool. Dissolve the wasabi in the sauce before serving. Yield: 1 1/2 cups.
A Ginger Soy dipping sauce would be perfect, but thats going to be thin.
Ginger Soy Dipping Sauce
1/4 Cup Soy Sauce
1/4 Cup Rice Vinegar
1/4 Cup Water
2 Tsp. Sugar
2 Tsp. Fresh Ginger peeled and grated
1/2 Tsp. Crushed Hot Red Pepper Flakes or moredepending on your taste.
2 Tsp. Sesame Oil, Asian
2 Green Onions, sliced thin
2 Tab. Chopped Cilantro, fresh
Combine all ingredients except the green onions and cilantro in a bowl and wisk to incorporate it all together. Stir in green onions and chopped cilantro.
I can picture what you want, I think, and this is close, but you're right - it needs to be thicker. I think the texture you want would from cornstarch, which almost makes it like a glaze. So, to that recipe, I'd whisk in cornstarch, then simmer it until thickened. You'd have to keep trying to get the right amount, but I'd start with a teaspoon.