Finely mince the red onion and scallions. Place them in a small bowl and sprinkle with a pinch of the sea salt; let sit 5 minutes, then squeeze lightly to remove excess sharpness and moisture.

Mix dressing

In a medium bowl whisk together the Dijon mustard, extra-virgin olive oil, lemon juice, soy sauce, sesame oil, remaining sea salt, and black pepper until emulsified.

Combine

Add the diced tuna, drained minced red onion and scallions, chopped capers, minced cornichons, and chopped parsley to the bowl with the dressing. Gently fold with a spoon until tuna is evenly coated. Taste and adjust seasoning with more lemon, salt, or pepper as desired. Cover and chill 10–15 minutes to let flavors meld (optional; can serve immediately).

To Serve

Prepare avocado

Halve, pit, and dice the avocado. If you prefer slices, thinly slice instead. Lightly toss the avocado with a few drops of lemon juice and a pinch of salt to prevent browning.

Assemble

Arrange the toasted baguette slices or crostini on a platter. Spoon about 1 to 1 1/2 tablespoons of the tuna mixture onto each crostini (or create small quenelles on a serving plate). Garnish each portion with a few microgreens or a small mound of mixed greens and a sprinkle of toasted sesame seeds if using.

Finish

Serve immediately with extra lemon wedges on the side for squeezing. Store any leftover tuna mixture covered in the refrigerator and consume within 24 hours.