Learn how to make Ludo Lefebvre's easy ceviche recipe. Slideshow: More Seafood Recipes
Gently toss the sliced fresh fish and lemon juice in a large bowl then set the mixture aside for 30 minutes, or until the fish is opaque. Drain the lemon juice and refrigerate until cold.
Blend the cucumbers in a high-powered blender until smooth. Strain the cucumber mixture in a fine-meshed sieve to extract the cucumber water; discard the solids. You should have about 2 cups of green cucumber water. Mix the cucumber water, balsamic vinegar, and champagne vinegar in a medium bowl to blend. Season the cucumber liquid to taste with salt and pepper, then whisk in the xanthan gum to thicken slightly. Refrigerate until cold.
Cook the red onions in a small saucepan for 30 seconds, then drain and quickly submerge the onions in a bowl of ice water until the onions are cold. Strain the onions then transfer them to a small bowl. Add the vinegar and marinate the onions for 1 hour. Strain the onions again, then cover and refrigerate.
Arrange the fish slices over the bottom of 6 shallow soup plates. Spoon about 5 tablespoons of cucumber water over the fish on each plate and scatter the pickled red onions (about 4 per plate) over the fish. Garnish with the jalapeño slices, cilantro leaves, purslane, and borage flowers. Season with a touch of fleur de sel and serve immediately.