Preheat an oven. Pat the fish dry with paper towels, then season on both sides with Kosher salt and ground black pepper. Place the seasoned fillet inside a baking dish, then drizzle with olive oil. Sprinkle on the reserved lemon zest.
In a medium bowl, combine halved tomatoes, olives, thinly sliced red onion, and garlic cloves. Drizzle with olive oil and red wine vinegar, then season with dried oregano and Kosher salt. Toss well, then pour on top of the cod.
Bake the fish until the internal temperature reaches around 140F (this will continue to rise as it rests). Serve immediately.
Serve with your favorite roasted or blistered vegetables for a healthy weeknight dinner.