Preheat the oven to 220°C. Line a large sheet pan with parchment paper. Use a wide sheet pan so the potatoes and beans roast instead of steaming. A genuinely wide pan gives the vegetables enough room to sit in a single layer with real airflow around them — a crowded pan traps steam between the pieces, softening them rather than letting them properly roast and develop crisp edges.

Add the halved baby potatoes to the sheet pan. Toss with olive oil, crushed garlic cloves, lemon slices, oregano or thyme, salt, black pepper, and parsley. Arrange the potatoes cut-side down where possible. Roast for 25 minutes. The potatoes need a real head start. If the cod goes in from the beginning, the fish will be dry before the potatoes are tender. Placing the cut side down, directly against the hot pan, gives that flat surface the most direct contact available for developing real, deep colour and crispness.

While the potatoes roast, pat the cod fillets dry with paper towels. In a small bowl, mix olive oil, lemon juice, lemon zest, minced garlic, oregano or thyme, salt, black pepper, chili flakes if using, and parsley. Spoon this mixture over the cod and let it sit while the potatoes finish their first roast. Dry cod plus a short lemon-garlic coating gives better texture. Do not marinate cod for a long time in lemon juice or the surface can turn mushy. Lemon juice's own acidity begins actively breaking down the fish's delicate protein structure the moment it makes contact — a brief coating flavours the surface without giving that acid enough time to compromise the cod's own texture the way an extended marinade would.

After the potatoes have roasted for 25 minutes, remove the tray from the oven. Add the green beans around the potatoes and toss them lightly in the oil and pan juices. Spread everything back out into an even layer. Return the tray to the oven for 8 to 10 minutes. Adding the beans only at this later stage, rather than alongside the potatoes from the start, respects their genuinely different cooking time — beans roasted the full 25 minutes alongside the potatoes would turn limp and overdone well before the potatoes were ready.

Remove the tray again. Make space between the potatoes and beans, then place the cod fillets on the sheet pan. Spoon any remaining lemon-garlic oil over the fish. Place a lemon slice on each fillet if you want the classic roasted lemon look. Return to the oven and bake for 10 to 12 minutes, depending on thickness. The cod is done when it flakes easily but still looks moist in the center. Do not wait until it looks dry and fully separated into flakes. Making genuine space for the cod, rather than tucking it in around already-cooking vegetables, gives the fish its own direct exposure to the oven's heat rather than cooking unevenly in whatever gaps happen to be available.

Remove the sheet pan from the oven and rest for 3 minutes. Spoon a little of the lemony pan juice over the cod. That brief rest lets the cod's own carryover heat finish gently, while spooning the accumulated pan juices back over the fish returns real flavour that collected on the tray during roasting.

Serve each portion with 1 cod fillet, about 250g roasted baby potatoes, about 150g green beans, roasted lemon slices, garlic cloves, fresh parsley, and lemon wedges. This dish is best delivered the moment it comes together, while the potatoes are still crisp-edged and the cod is at its most moist.