Place julienned vegetables in a resealable jar. Heat water in microwave for about 2 minutes until boiling. Add sugar, salt, and rice vinegar, then stir until the sugar and salt is dissolved. Pour over vegetables in jar and allow to pickle overnight.
Place halibut in a shallow bowl and top with soy sauce, rice vinegar, and lime juice. Flip halibut so that all sides are coated with sauce. Marinate for 30 minutes in the refrigerator.
Heat a non-stick pan over medium-high heat. Add a small amount of sesame or olive oil to the pan if desired, then place halibut in pan and cover immediately. Cook for about 5 minutes before flipping and continuing to cook through on the other side until fish flakes apart easily. (Your cooking time will depend on the thickness of your fish - mine was about 2" thick and took about 10 minutes total.)
Mix sriracha mayo ingredients until combined and set aside.
Toast tortillas in a shallow pan or on a griddle until heated through and slightly browned.
Layer tortillas with fish, pickled vegetables, basil/mint/cilantro, and sriracha mayo. Serve immediately.