For the sauce: In a medium bowl, mix together the mayonnaise, honey, soy sauce, lemon zest and juice and wasabi. Taste--it should be salty, sweet and a bit tangy. Balance the seasoning if needed. Refrigerate.
For the tuna: In a large bowl, mix together the tempura batter, cold seltzer and soy sauce. You want the batter to have a pancake-like consistency; you may not need all the seltzer. Set aside in the refrigerator.
Slice the tuna against the grain into 3/4- to 1-inch-thick sticks. You should be able to make 4 to 5 sticks from each tuna steak. If using skewers (not needed), push one down the center of each. Dust each tuna stick all over with salt and pepper, then some flour. Transfer to a baking sheet lined with parchment paper and freeze for 30 minutes.
Meanwhile, add enough oil to a pot to deep-fry the fish sticks and heat to 375 degrees F.
When ready to fry, remove the batter from the refrigerator and the floured tuna from the freezer. Check all sides to make sure the flour is still dusty and not soaked in; sprinkle with flour again if needed, then dip into the batter and fry until golden, about 2 1/2 minutes. Remove and season immediately with everything bagel seasoning or a pinch of salt. Serve with the dipping sauce.