Pre-heat the oven to 375º/
Line 2 baking trays with baking paper. Cut the potatoes into halves and put in a bowl with 2 tbsp of the oil. Toss to coat in oil. Add salt and pepper to taste and toss again.
Spread out the potatoes on one of the baking trays. Cook in the oven for 30 minutes or until tender and browned at the edges, turning them over halfway through.
Brush the fish fillets on both sides with the 2 tbsp of oil and sprinkle evenly, again on both sides, with salt and pepper. Transfer on the other baking tray and cook in the oven for 10 minutes or until the fish flakes easily.
Carefully transfer the cod into the potato tray add the green beans, garlic, lemon juice, and drizzle with a little olive oil.
Transfer back to the oven and bake for an additional 10 minutes.