Prepare the gremolata by finely chopping the fresh parsley and combining it in a small bowl with the lemon zest, minced garlic, and a pinch of sea salt.
Pat the butterflied sardines completely dry with a paper towel and season both sides evenly with sea salt and black pepper.
Heat the extra virgin olive oil in a large cast-iron or stainless steel skillet over medium-high heat until the oil is shimmering but not smoking.
Carefully place the sardines in the skillet, skin-side down, and sear for 2-3 minutes without moving them to ensure the skin becomes golden and crispy.
Gently flip the sardines using a thin spatula and cook for an additional 1-2 minutes until the flesh is opaque and cooked through.
While the fish finishes, toast the sourdough bread until it is firm and lightly browned.
In a small bowl, toss the baby arugula with the lemon juice and a tiny drop of olive oil to lightly coat the leaves.
Assemble the dish by placing the dressed arugula on the sourdough toast, layering the seared sardines on top, and finishing with a generous spoonful of the lemon-herb gremolata.