Cut spring onions into slices, fennel into small cubes. Chop garlic finely and basil into fine stripes. Wash shrimps and let them drain well.
Heat oil in a pot, fry onions and garlic gently, add fennel and stew for some minutes. Spice with salt and pepper. Fold in saffron, add beef stock and let all simmer for 10 minutes.
Cut fish fillets into 3 cm cubes, sprinkle with lemon juice and add them into the pot. Cook for 10 minutes again. After 5 minutes add shrimps and stir in mascarpone cheese. Season to taste with salt and pepper and thicken the sauce.