Make the Thai green sauce: Add the Thai basil, lime juice, fish sauce, sugar, garlic, and chili to a food processor, blender or mortar and pestle. Blend until smooth, scraping down the sides as needed. If the sauce is too thick, add 1 to 2 tablespoons of water until it reaches a pourable consistency. Taste and adjust with additional lime juice or fish sauce if needed.
Slice the tuna: Using a very sharp knife, slice the sushi-grade tuna against the grain into thin, even slices about ¼-inch thick.
Assemble the tuna: Arrange the tuna slices in a single layer on a chilled serving platter, slightly overlapping them. Spoon or drizzle the Thai green sauce generously over the tuna.
Finish: Sprinkle with flaky sea salt and garnish with sliced chilis, Thai basil leaves, lime zest, toasted sesame seeds, chili crisp, or microgreens if desired.
Serve: Serve immediately with toasted baguette slices, crispy wonton chips, or enjoy on its own as a light appetizer.