Peel and dice the cucumber. Place in a bowl and season with salt and sugar. Let stand for 30 minutes so the cucumber releases water. Use a hand blender to finely chop the mixture, then strain through a cloth and collect the juice.
Soak the gelatin sheets in cold water for 5 minutes until softened, then gently squeeze out the excess water. Measure 250 ml of the cucumber juice and stir in the vinegar. Season well with salt and sugar. Heat half of this mixture (about 125 ml) over medium heat to about 60°C (140°F). Dissolve the softened gelatin in the hot juice, then stir in the remaining mixture. Pour into a bowl and refrigerate overnight (at least 12 hours) until set.
Cut the salmon fillet into thin slices and place on a baking sheet. Use a kitchen torch to sear the surface of each slice for about 10 seconds. Season with lime juice, fleur de sel, and freshly ground black pepper.
Place some cucumber jelly on each plate, top with the seared salmon slices, and serve immediately.