For the onion sauce, place a large saucepan over a medium-high heat and add the oil. Once the oil is hot, add the onions and cook until well browned, stirring occasionally Remove the browned onions and drain on paper towel. Add the browned onions to a clean saucepan along with the fish or vegetable stock and freshly ground black peppercorns. Simmer for 15 minutes, then strain into a small saucepan and set aside
Flash-fry the leek slices in hot oil until golden brown, then remove and drain on paper towel. Set aside until required
To prepare the cod, cut the fillet into 4 portions and set aside. Dry-roast the spices in a pan, then grind in a spice-grinder (or using a mortar and pestle) to a coarse powder. Sprinkle the spice powder onto the cod fillets and set aside
Preheat the oven to 200°C/gas mark 6
Place a non-stick pan over a medium-high heat and add the butter. Season the spiced cod with salt and cook in the pan for 2 minutes on each side
Line a baking tray with parchment paper and carefully transfer the seared cod to the tray. Add the asparagus and shiitake mushrooms, drizzle with olive oil and finish cooking in the oven for 4-5 minutes, or until the cod is cooked to your liking Meanwhile, bring the onion sauce to the boil. Mix cornflour with a teaspoon of cold water and add to the boiling sauce to thicken, stirring continuously. Season with salt and set aside.
To serve, place each fillet of cod in the middle of a soup plate and arrange the asparagus and mushrooms. Pour over the onion sauce and garnish with the crispy fried leeks. Serve immediately.