Clean and wash the sugar snap peas. Cook ribbon noodles in boiling salted water for 8-10 minutes. Add sugar snap peas to the noodles about 5 minutes before the end of the cooking time. Bring 1/8 litre salted water and lemon juice to the boil. Cut salmon into large pieces and add to the water. Let it simmer at low heat for about 4 minutes. Take the fish out and keep warm.
Add cream to the fish stock and bring to the boil. Stir in the sauce thickener and bring to the boil again. Wash tarragon, dab dry, pluck leaves from the stems and chop finely, except for a few for garnishing. Add to the sauce and season with salt and pepper. Drain the pasta and mangetouts and serve with the salmon ragout. Garnish with remaining tarragon