Preheat the oven to gas 5, 190C, fan 170°C Line a large baking tray with baking parchment and rub with a little oil. Slice the potatoes to the thickness of a pound coin and place them on the tray (don't overlap). Sprinkle with salt, freshly ground black pepper and oregano and drizzle with 2 tablespoons of oil. Put in the oven for 15 minutes.
Remove the tray and turn up the oven to gas 7, 220C, fan 200°C Lay the fillets on the potatoes, scatter over the veg, and drape the ham on top. Drizzle over the remaining oil and cook for 10-15 minutes, until the salmon is done. Squeeze over the lemon juice; serve.