Preheat your oven to 400°F and set a baking sheet with a piece of parchment paper.
In a shallow bowl, mix the panko breadcrumbs with lime zest, cumin (if using), salt, and pepper.
Lightly whisk the egg white in another bowl. Pat the cod fillet dry and brush with a little lime juice.
Dip the cod fillet into the egg white, then coat evenly with the breadcrumb mixture.
Place the coated fish on the prepared baking sheet and bake for 12-15 minutes until the fish flakes easily with a fork.
While the fish bakes, prepare the slaw by tossing the shredded cabbage with a squeeze of lime juice, a pinch of salt, and pepper.
Warm the whole wheat tortillas in a dry skillet or microwave for a few seconds until pliable.
Assemble the tacos by flaking the baked fish into bite-sized pieces, placing them on the tortillas, and topping with the cabbage slaw and sliced avocado.
Drizzle a little extra lime juice over the tacos if desired, and serve immediately.