In a medium bowl, mix the cucumber with the crème fraîche and 2 Tbs. of the lemon juice. Add the dill and season to taste with salt.
In a large bowl, whisk the remaining 2 Tbs. lemon juice with the olive oil and a pinch of salt. In the bowl, flake the trout into chunks, making sure to remove any bones. Add the scallions and gently toss to combine. Season to taste with salt and pepper.
Make a bed of the cucumbers on a large platter (or divide them among 8 small plates); top with the trout mixture. Garnish with a dill sprig and serve with the toast.
nutrition information (per serving):
Photo: Pernille Pedersen