Preheat your oven to 425°F and line a large baking sheet with parchment paper for easy cleanup.
Slice the Yukon Gold potatoes into very thin rounds, approximately 1/8-inch thick, to ensure they bake rapidly and get crispy.
In a medium bowl, toss the potato slices with 0.5 tbsp of the olive oil, 0.25 tsp of dried oregano, and a pinch of sea salt and black pepper.
Spread the potatoes in a single layer on the prepared baking sheet and roast for 15 minutes.
While the potatoes are roasting, pat the cod fillets dry with a paper towel and season them with the remaining olive oil, garlic powder, and the rest of the salt, pepper, and oregano.
Remove the tray from the oven, slide the potatoes to one side, and place the cod fillets on the other side of the pan.
Place thin lemon slices on top of each cod fillet and return the tray to the oven for 10 to 12 minutes until the fish is opaque and the potatoes are tender.
Garnish the entire medley with freshly chopped parsley and serve immediately while hot.