Wash the fish, dab dry and cut into four pieces. Sprinkle with lemon juice, season with salt and pepper and fry on both sides in 2 tablespoons of hot oil until golden brown. Cook the pasta in plenty of boiling salted water for 8-10 minutes.
Peel and finely dice the onion and sauté in the remaining oil. Add fish stock and cream, stir in tomato paste and saffron and bring to the boil. Stir in sauce thickener. Season sauce with salt and pepper.
Wash, clean, quarter and cut the tomato into small cubes. Wash tarragon, dab dry and chop coarsely. Add tomato cubes and tarragon to the sauce just before serving. Arrange fish with pasta and sauce on a plate.
Garnish with lemon wedges.