In a small bowl, stir together the onion, vinegar and ½ tsp salt; set aside. Fill a large Dutch oven with about 1" of water. Place a folding steamer basket in the pot, cover and bring to a boil over medium-high. Add the eggs, cover and steam for 9 minutes. Meanwhile, fill a medium bowl with ice water.
Immediately transfer the eggs to the ice water and let stand until cooled. Crack and peel the eggs. Cut 9 of the eggs lengthwise into quarters, then cut the quarters crosswise into thirds. Transfer to a medium bowl. Add ¾ tsp salt and ½ tsp pepper and gently toss. Halve the remaining 3 eggs and transfer the yolks to a small bowl; roughly chop the whites and add them to the bowl with the chopped eggs.
Set a fine mesh strainer over the bowl containing the yolks and pour the onion mixture into the strainer; set the drained onions aside. Using a fork, mash the yolk-vinegar mixture until smooth, then whisk in the turmeric.
Sprinkle the tarragon, chives and parsley over the chopped eggs and add the yolk mixture. Fold until just combined; do not overmix. Taste and season with salt and pepper. Transfer to a serving bowl and sprinkle with the reserved onions. Serve with lemon wedges and flaky salt.
