Heat the oven to 425 degrees. Toss the lemon slices with 1 tablespoon of oil and ⅛ teaspoon of salt on a small parchment-lined baking sheet, and arrange in a single layer. Roast for about 12 minutes, until the lemon is tender and just starting to char around the edges. Set aside to cool briefly, then finely chop.
In a medium bowl, combine the cucumbers, gherkins, pickle brine, spring onion, dill and jalapeño. Stir in the chopped roasted lemon, along with the remaining 5 tablespoons oil, the maple syrup, celery salt and a few grinds of pepper.
In a separate bowl, mix together the chickpeas, yogurt, mayonnaise and mustard, along with ¼ teaspoon of salt and a few grinds of pepper.
To serve, transfer the chickpeas to a serving platter, spoon over the cucumber salsa and sprinkle with the nigella seeds, if using.
