Give the tomatoes a head start
A shallow baking dish lets cherry tomatoes soften and release juice before the cod goes in. Keep the tomatoes whole so that some collapse into the sauce while others retain their shape. Thin onion slices cook alongside them, rather than sitting in a dense raw layer underneath the fish. Choose a dish large enough to hold four portions without overlapping.
The first oven interval is also a useful moment to check the fish. Pat each portion dry, remove any remaining pin bones with clean tweezers and keep the cod refrigerated until the tomato base is ready. Pieces of similar thickness make the final cooking window easier to manage. A thin tail piece will usually finish ahead of a thick center cut.
Let olives supply part of the seasoning
Pitted black olives add briny flavor without needing a separate sauce. Drain them well and halve them so they distribute through the tomato juices. The measured salt is intentionally modest because olive saltiness varies. Keep the dried oregano in the vegetable base and add the fresh parsley after baking, so it keeps its fresh aroma.
Turn the tomatoes once after their first roast, then nestle the fish into the hot vegetables. Spoon a little tomato juice over the top, keeping most of the white flesh visible. The fish is baked uncovered. A heavy blanket of vegetables would make it harder to check the thickest part and could change the cooking time.
Check the center, then serve promptly
Use a thermometer in the thickest part of every portion and cook to 145 F (63 C). Flaking and an opaque appearance are useful supporting observations, but the temperature gives a clearer endpoint when pieces differ in size. Remove finished portions while any thicker pieces complete cooking.
Serve the fish with its tomato and olive juices. A broad spatula helps lift the delicate portions without breaking them. Bread or a cooked grain can accompany it, but those additions are separate from the ingredient quantities and estimated time here. Spoon the vegetables beside the fish so each plate has both tomato sweetness and briny olive pieces.
