I came across these recipes for sushi, one of which uses vegan tuna sashimi. Basically, it's the fleshy part (minus seeds/pulp) of Roma tomatoes marinated in 1 T soy sauce, 1 T water, 1 T rice vinegar, 1 t maple syrup, 1 t toasted sesame oil, 1 T sea vegetable blend (no written recipe, unfortunately, so I had to transcribe).
He called the sea vegetable blend "optional" but I don't see how it has the chance to be tuna-like without it. That said, though it does look like it's topped with tuna, is this basically just going to taste like tomato, marinade or not? I'm not sure it's worth the effort to just approximate appearance and not taste–especially as I haven't yet found the sea veg blend locally and would have to order it.
by Scott_A_R