In a small bowl whisk together 3 tbsp lemon juice, 2 tbsp olive oil, 2 tbsp melted unsalted butter, minced garlic, 1 tsp salt, 1 tsp black pepper, and 1 tsp dried oregano until combined.

Pour or brush about two-thirds of the marinade evenly over both sides of the fillets. Let sit for 10–15 minutes at room temperature to absorb flavors (while preparing vegetables). Reserve remaining marinade for later.

Prepare vegetable topping

In a medium bowl combine diced tomatoes, sliced onion, and sliced bell pepper. Add 0.5 tsp salt, 0.5 tsp black pepper, 0.25 tsp red pepper flakes (if using), and 2 tbsp chopped parsley. Toss gently to combine.

Add the remaining reserved marinade to the vegetable mixture and toss again so the vegetables pick up the lemon-garlic flavor. Taste and adjust seasoning if needed.

Assemble and bake

Lightly oil a baking dish large enough to hold the fillets in a single layer (about 9x13 inches or similar). Place the marinated fish fillets in the dish, skin-side down if skin is present, leaving small gaps between fillets.

Divide the vegetable mixture evenly over the top of each fillet, mounding slightly so the fish is well covered. Tuck thin lemon slices among the vegetables and fillets for extra brightness.

Drizzle the 1 tbsp melted butter over the vegetables and fish for added richness.

Cover loosely with foil and bake in the preheated oven for 12–15 minutes, depending on fillet thickness (about 10 minutes per inch of thickness). Remove foil and bake an additional 3–5 minutes to lightly brown the vegetables and finish cooking. Fish is done when it flakes easily with a fork and reaches an internal temperature of 145°F (63°C).

Finish and serve

Remove the dish from the oven. Sprinkle 1 tbsp chopped parsley over the top and squeeze additional lemon juice if desired. Serve each fillet with a lemon wedge.

If desired, spoon pan juices over the fish and serve immediately with a side of rice, roasted potatoes, or a simple green salad.