Drain tofu and use your favorite method to press for at least 30 minutes. (This step is important; if you don't press out excess liquid, your salad will become watery.) See notes for tips on pressing tofu.
Add half of the block of tofu to a mixing bowl. Add the vegan mayo, mustard, and optional turmeric. Mash everything together with a fork to create a thick dressing. (Mashing some of the tofu makes the salad more cohesive, so it's less likely to fall out of your sandwich.)
Give the rest of the block of tofu a coarse chop to mimic the texture of chopped egg whites. Add the tofu to the mixing bowl along with all remaining ingredients (celery, green onion, dill, black salt, and black pepper). Fold all ingredients together.
Give the salad a taste and adjust the flavors to your preference. This recipe is very flexible, so feel free to add more fresh dill, extra Dijon, or even a squeeze of fresh lemon juice if you like.
You can serve it immediately, but it's best if you let it marinate for an hour or two in the fridge.