Preheat the oven: Set to 400°F (200°C). Line a large baking sheet or shallow baking dish with parchment for easy cleanup.
Prep the veggies: In the dish, combine tomatoes, olives, red onion, garlic, capers (if using), lemon zest, and half the parsley.
Drizzle with 1–2 tablespoons olive oil, season with salt, pepper, and oregano. Toss to coat and spread in an even layer.
Season the fish: Pat fillets dry. Rub with 1 tablespoon olive oil, a big pinch of salt, pepper, and a light sprinkle of red pepper flakes if you like heat.
Nestle and bake: Place the fish on top of the tomato-olive mixture.
Squeeze the lemon juice over everything. Bake 10–14 minutes, depending on thickness, until the fish flakes easily with a fork and is opaque in the center.
Finish with freshness: Sprinkle remaining parsley over the top. Taste the pan juices and add a pinch of salt or a squeeze of lemon if needed.
Serve: Spoon the tomatoes and juices over the fish.
Add a handful of fresh spinach or arugula to each plate and let the warm juices lightly wilt the greens. Serve with whole-grain bread or brown rice if you want a heartier meal.