Preheat the oven to 450 degrees . On a large baking sheet, toss the onion, sweet potato, squash, garlic and 2 tbsp. oil. Arrange in a single layer; season with salt and pepper. Roast, rotating the sheet halfway through, until the vegetables are tender and browned, about 25 minutes. Remove from the oven; top with the spinach. Drizzle with 2 tbsp. oil; season and toss until the spinach wilts slightly, about 2 minutes.
Sprinkle the oregano on both sides of the cod fillets; season. Place the flour in a bowl and dredge each fillet to coat.
In a large nonstick skillet, heat the remaining 1 tbsp. oil over medium-high. Add the cod and cook, turning once, until the fillets are golden brown on the outside and just opaque in the centers, about 5 minutes per side. Serve with the vegetable mixture.