Mix together the salt, sugar, dried minced garlic, dried minced onion, poppy seeds, sesame seeds and zest. Place the fish, skin-side-down, on a long piece of plastic wrap. Splash with the vodka, and then pack the salt mixture on top of the exposed flesh. Sandwich the fillets together, wrap well in plastic wrap, place on a baking sheet or baking dish and refrigerate for about 1 day. Remove the fish from the wrap, rinse well and pat dry. Slice the gravlax very thinly at an angle leaving the skin behind (see Cook's Note). Serve on bagel chips topped with sour cream or cream cheese. Garnish with poppy seeds, sesame seeds and sliced scallions.
To make nice thin slices of gravlax, lay it skin-side down on a cutting board. Use a long, very sharp, very thin carving knife, preferably with a hollowed blade. Slice at a low angle from the skin, about 30 degrees up from the table, so that the slices are nice and broad. When slicing, only apply downward pressure when you push the knife, and not when you pull it toward you. Leaving the skin on helps with cutting. Place the flat of your hand on the other side of the slice you are cutting. (Be careful.)