Preheat oven to 375F. Line a rimmed baking sheet with parchment paper, then place cod fillets on top. 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 145F. Set aside until cool enough to handle fish with your hands, about 5-10 minutes.
Meanwhile, melt butter in a skillet over medium-high heat. Add garlic and onion and sauté until translucent, about 3-4 minutes. Set aside.
In a large bowl, whisk together mayonnaise, dijon mustard, eggs, Old Bay, 1 ½ tsp kosher salt, and ¼ tsp black pepper. To this add sautéed onion and garlic, panko breadcrumbs, fresh herbs, 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.
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".
Heat grapeseed (or other 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, some additional fresh herbs for garnish, and a sprinkling of flaky sea salt if you've got it!