Preheat the oven to 180°C/fan 160°C/gas mark 4. Place the cashews in the preheated oven for 5-8 minutes or until warmed through.
While the cashews are warming, bring a pan of water with a steamer basket to the boil, then turn down to a simmer. Season the cod fillets with salt and pepper, and drizzle lightly with half the oil. Do the same with the asparagus.
Place the cod into the steamer basket, cover and steam for 8–10 minutes (depending on the thickness). Once the cod is cooked, remove from the heat. Add the asparagus and spinach to the steamer, cover and steam for 3–4 minutes.
Place the warmed cashew nuts in a food processor and start to blitz, incorporating the rest of the ingredients as it runs, until smooth. Adjust the seasoning and consistency to your liking.
To serve, divide the asparagus and spinach between 4 plates, then top with the cod and the ‘hollandaiseʼ sauce.