It's always best to use fresh vegetables, but if necessary you could use canned or frozen vegetables. 

This recipe can be made in advance. It's excellent cold: let the trout and vegetables chill in the court-bouillon. 

Preparing the vegetables in court-bouillon

1. place all the ingredients for the court-bouillon into a large saucepan; boil gently, covered, for 20 minutes; correct the seasoning. 

Preparing the trout

1. gut and clean the trout; remove the gills and fins; wash under cold water;
2. place into a small heatproof dish or fish poacher; 
3. pour the hot court-bouillon over the trout and maintain at a simmer; 
4. poach the trout for 10 minutes; remove gently with a spatula; remove the skin, beginning at the tail end and pulling slowly towards the head; 
5. ladle some court-bouillon that has been flavored with the juice of 1 lemon into each serving bowl; divide the vegetables among the bowls and lay the trout fillets on top; 
6. garnish with lemon slices, parsley and tarragon.