Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Scrub the potato and dice into 1/2-inch cubes, then toss in a bowl with 0.5 tbsp olive oil, 0.25 tsp salt, and 0.5 tsp garlic powder.
Spread the potatoes in a single layer on the baking sheet and roast for 15 minutes.
While potatoes roast, pat the cod dry and season both sides with the remaining olive oil, oregano, garlic powder, salt, and pepper.
Remove the tray from the oven, slide potatoes to one side, and place the cod and trimmed asparagus on the other side.
Return to the oven and bake for 12 to 15 minutes until the fish flakes easily with a fork and the potatoes are golden.
Finish with a bright squeeze of fresh lemon juice and a sprinkle of chopped parsley before serving.