Line baking tray with baking paper. Rinse fish, dab dry and place on baking tray. Cut in about every 3 cm crosswise, but do not cut through. Halve the lemon and squeeze the juice. Mix lemon juice, Garam Masala, chilli powder and 1⁄2 tsp. salt.
Spread the marinade over the fish and set aside covered for about 30 minutes.
Preheat oven (electric cooker: 150 °C/circulating air: 125 °C/gas: see manufacturer). Sprinkle the salmon with semolina. Bake in the hot oven for about 30 minutes. Peel and wash the potatoes and cut them roughly into pieces.
Cover the potatoes and cook in salted water for about 20 minutes.
Drain the ginger. Peel onions. Cut 1 onion into fine strips. Wash parsley, shake dry and chop coarsely. Mix onion strips, parsley and ginger.
Clean and wash spring onions and cut into thin rings. Finely dice the remaining red onions. Peel garlic and chop finely. Heat 1 tbsp. butter, fry diced onions and garlic for approx. 4 minutes.
Drain the potatoes. Add milk and 2 tablespoons of butter and mash to a puree. Fold in spring onions and fried onions. Season to taste with salt. Cut salmon into 4 pieces and arrange on plates with mashed potatoes.
Add the ginger, parsley and onion mixture.