In bowl, toss together cucumbers, onion, vinegar, 1 T. sugar and 1 t. salt. Season with additional salt and pepper to taste. Let stand for 1 hour at room temperature.
In a bowl, with the back of a spoon, mash ginger and garlic, chile and remaining 1 t. sugar to make a paste.
Stir in fish sauce, cilantro and basil.
On a clean cutting board, thinly slice tuna. Stack slices and cut into thin matchsticks.
Then cut matchstick tuna into rough cubes and then into smaller cubes.
Add tuna to ginger paste and stir to blend well.
With moist hands, shape tuna mixture into 4 patties about 1” thick. Set on plate lined with plastic wrap and refrigerate for about 30 minutes.
Light a grill. In bowl, combine vegetable and sesame oil. Lightly brush burgers and cut side of buns with oil.
Grill burgers for 6 minutes, turning once, for medium rare. Move burgers away from heat and grill cut sides of buns until toasted.
Drain cucumber salad. Spread the spicy mayo or mustard on cut sides of bun and set burgers on the bottoms.
Top with cucumber salad. Cover with top bun and serve.
Recipe by Geaux Ask Alice! at https://geauxaskalice.com/tangy-tuna-burger/