Bake the cod. Preheat oven to 375°F (190°C). Line a rimmed baking sheet with parchment paper, then place cod fillets on top. Pat dry with a paper towel, then drizzle with 2 Tbsp olive oil, 1 tsp Kosher salt, and ¼ tsp freshly ground black pepper. Cook for 12 minutes, or until fish is an internal temperature of 145°F (63°C). Set aside until cool enough to handle fish with your hands, about 5-10 minutes.
Sauté the onion. Meanwhile, melt 3 Tbsp butter in a skillet over medium-high heat. Add diced onion and sauté, stirring occasionally, until translucent, about 3-4 minutes. Add chopped garlic cloves, then cook 1 minute more. Set aside.
Make the fish cake mixture. In a large bowl, whisk together ¼ cup mayonnaise, 1 Tbsp dijon mustard, 2 eggs, 1 Tbsp Old Bay, 1½ tsp kosher salt, and ¼ tsp black pepper. To this add sautéed onion and garlic, 1½ cups panko breadcrumbs, 2 Tbsp chopped parsley, 2 Tbsp chopped dill, 1 Tbsp sliced chives, and lemon zest. Flake the slightly cooled fish with your fingers or two forks, being careful to keep the flakes as large as possible. Gently combine all ingredients together until homogenous and evenly mixed. Place fish cake mixture in a refrigerator to cool completely, about 1 hour.
Form the fish cakes. When ready, use a large cookie scoop to form equal sized patties. Press together gently in your hands to flatten and round each out to about a height of 1-inch.
Pan fry, then enjoy. Heat 2 Tbsp neutral oil over medium-high heat in a large skillet or on a griddle. When hot, add fish cakes and cook 3-5 minutes per side, being careful to flip each gently so they don't break apart. Serve on top of arugula or mixed greens with lemon wedges, tartare sauce, additional fresh herbs, and a sprinkling of flaky sea salt.