Pat the cod fillets dry with paper towels. Season both sides evenly with salt, black pepper, and smoked paprika.

Set up a breading station with three shallow bowls: one with flour, one with beaten eggs, and one with panko breadcrumbs.

Dredge each cod fillet first in the flour, shaking off excess. Dip into the egg, letting any excess drip off. Then coat thoroughly with panko breadcrumbs, pressing gently so the crumbs adhere.

Heat about 1/2 inch of vegetable oil in a large, heavy skillet over medium-high heat until shimmering (around 350 degrees F). Carefully place the breaded fillets in the hot oil and fry for 3 to 4 minutes per side, until golden brown and the fish flakes easily with a fork. Transfer to a plate lined with paper towels to drain.

If desired, lightly toast the sandwich buns on a griddle or under the broiler. Stir the Meyer lemon zest into the tartar sauce (if using). Spread a generous tablespoon of tartar sauce on the bottom half of each bun.

Layer each bottom bun with a lettuce leaf, a fried cod fillet, a tomato slice, and a few slices of avocado (if using). Cap with the top bun and serve immediately while hot and crispy.