Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Slice the zucchini into rounds, the bell pepper into strips, and the red onion into wedges.
Place the vegetables on the baking sheet and toss with 1 tablespoon of olive oil, dried oregano, sea salt, and black pepper.
Spread the vegetables in a single layer and roast in the oven for 10 minutes to start the softening process.
In a small bowl, whisk together the remaining 0.5 tablespoon of olive oil, lemon juice, lemon zest, and minced garlic.
Remove the sheet pan from the oven, move the vegetables to the edges, and place the cod fillets in the center.
Brush the lemon-garlic mixture generously over the top of the fish fillets.
Return the pan to the oven and bake for 12 to 15 minutes, or until the fish is opaque and flakes easily with a fork.
Sprinkle the entire dish with fresh chopped parsley before serving warm.