Season each salmon fillet with salt and pepper; set aside.
Melt 15g butter in a large frying pan over medium heat. Stir in onions and cook until they soften and turn translucent, 5 to 7 minutes. Add garlic and cook 1 minute more. Combine mushrooms, fennel and celery with onions and cook until vegetables have softened, about 5 to 7 minutes. Season with curry powder, saffron, salt and pepper to taste; cook 2 more minutes. Pour in chicken stock and cook 5 minutes longer, stirring occasionally. Stir in double cream and simmer 5 minutes.
Meanwhile, melt 15g butter in a large frying pan over medium heat and place salmon fillets in pan, skin side down. Turn up heat to high to sear fillets; cooking two minutes on each side.
Prepare serving plate by spooning mushroom sauce over the bottom. Place salmon fillets on top, drizzling additional sauce over fish. Garnish with parsley, lemon slices and spring onions.
Fresh porcini mushrooms
If your local supermarket doesn't sell these, you can buy them online or substitute in rehydrated dried porcini mushrooms. They are also known as ceps.