1. Spoon crème fraîche into a shallow pool on base of a serving platter (or divide among salad plates or shallow bowls). Halve cucumbers lengthwise and slice into 1/2-inch pieces. If you have found Mexican sour gherkins, keep the smallest ones whole and halve the others lengthwise.
2. Place all cucumbers in large bowl and season generously with fine sea salt. Add onions, lemon juice, Banyuls, and olive oil; toss to coat. Taste and adjust seasoning.
3. Add cucumbers to top of crème fraîche, letting the cucumbers tumble from your hands into a fairly even but haphazard layer. Drizzle some remaining vinaigrette all over. Sprinkle with capers and dill; dust with the dill pollen (if using) and flaky sea salt. Serve immediately.