Cut the onion in half. Then slice thinly, rinse under warm water, then add to a bowl.
In a small saucepan add water, rice vinegar, sugar and salt. Bring to a boil and pour the liquid on the onions and let sit for 15 mins on the counter to quick pickle wile you prep the rest of the salad.
Creamy Herb Dressing
Add the sour cream, mayonnaise, buttermilk, lemon juice, finely chopped shallot, herbs, garlic powder and salt into a bowl, mix well together and set aside.
Tuna Salad
Cut the cucumbers in half, scoop out the seeds with a spoon or a melon scooper, then slice diagonally into ½ inch pieces. Trim the celery ribs and cut them diagonally into ½ inch pieces. Cut the fennel in half, cut out the core and thinly slice.
Add the veggies into a mixing bowl, then add the lemon zest, rip the dill and tarragon into the bowl, chop the parsley and add in the bowl. Add the tuna from the jar and break it into smaller pieces. Add the remaining oil form the tuna, squeeze ¼ of lemon and add flaky sea salt. Gently toss together.
Assemble Salad
Add the creamy dressing to a serving platter. Spread into a thin layer. Top with the tuna salad and enjoy. Serve on it’s own or with toasted bread.