Combine all the ingredients together in a small bowl. Cover and refrigerate until ready to use. The sauce can be made a day or two ahead of serving.
Combine the halibut, buttermilk and 1 teaspoon of garlic powder together in a medium bowl. Cover and chill for 20 to 30 minutes, time permitting.
Place the cornflakes in a resealable plastic bag. Seal the bag and lightly crush the cornflakes using a meat mallet or a rolling pin until the cornflakes are broken up but not completely crushed.
Add the flour, the remaining garlic powder, salt and pepper and mix well. You can either pour the coating into a shallow dish or leave it in the bag for easy clean-up. Remove the pieces of halibut a few at a time from the buttermilk using tongs and drop into the coating. Press the halibut into the coating, making sure each piece is well covered (wearing disposable gloves makes for easy clean up). Transfer to a baking sheet or plate. Repeat with remaining halibut and coating. Discard buttermilk and any coating left (I never have coating left).
Preheat the oven to ‘keep warm’.
Cover the bottom of a large, non-stick frying pan with the oil. Heat the oil on medium. Cook the halibut in two or three batches, 2 to 3 minutes per side, until golden brown. Remove to a paper towel-lined baking sheet. Keep warm in the oven while you cook the next batch.
To assemble the tacos, spread the inside of each tortilla with some of the tartare sauce. Add lettuce, the halibut and onion then dollop with a little more sauce. Serve immediately with lemon wedges. Makes 8.