Put the haddock fillets into a shallow pan, add the milk and poach for 5 minutes, or until done. When cooked, strain, reserving the milk. Cover the haddock and keep warm.
Meanwhile, heat the oil in frying pan, add the shallots and garlic and fry gently for 5 minutes until soft. Add the red peppers and cook gently for 7—8 minutes, stirring occasionally.
Put the crème fraîche into a bowl, stir in 60 ml/2 fl oz of the strained milk, then the low-fat soft cheese, if using. Stir in the red pepper mixture (or purée it in a food processor if you would like a very smooth sauce). Return the mixture to the pan and add the tarragon. Season the sauce to taste with salt and pepper, then simmer for 10 minutes.
To serve, pour the sauce on to the plate and place the haddock on top. Sprinkle with pepper, garnish with tarragon and serve immediately.