Rinse flounder fillets in cold water, and pat dry with a paper towel.
In a small mixing bowl, gently whisk together the egg, cornstarch, lemon juice, and lemon zest.
In a shallow dish, combine the bread crumbs with the parsley flakes, kosher salt, and ground black pepper.
Dip the flounder fillets into the egg mixture and place them into the bread crumb mixture to coat.
Heat olive oil in a large skillet over medium heat until shimmering. Sauté the fillets for 3-5 minutes, turning once, until golden brown. Remove to a paper towel to drain.
Drizzle a small amount of olive oil onto the tomato slices, then sprinkle with kosher salt and pepper.
Lightly toast the Po' Boy bread. Add shredded lettuce or cabbage tossed in Tartar sauce, tomato slices, the flounder fillets, and a squeeze of lemon with a few dashes of Tabasco.