Meanwhile, melt the butter in a small saucepan and saute the scallion for 1-2 minutes, until soft. Add the peas and cook for 1 minute, then add the creme fraiche or cream and cook, stirring for 1-2 minutes, until the peas are cooked.
Open up the parchment and discard the lemon. Transfer the fish to a plate and pour the remaining juices into the saucepan. Stir the sauce and add 1-2tsp of milk, if needed, to take slightly thinner. Season to taste with salt and pepper ( from 1 year)
For smaller babies, the fish, peas and sauce can be mashed or pureed. Also, stir 1 tsp grated parmesan into the sauce instead salt or pepper.
Serve with rice or boiled baby potatoes.