Peel, deseed, chop, and salt the cucumbers (1 tsp). Leave 30 minutes, then pat dry. Chop finely or pulse briefly for a coarse texture.
Heat 2 tbsp oil and cook the onions for 8–10 minutes until soft.
Stir in garlic, ginger, mustard seeds, fennel, turmeric, cumin, and chilli flakes; cook 1–2 minutes until fragrant.
Add cucumbers, lemon zest and juice, sugar, and vinegar. Stir until the sugar dissolves.
Simmer gently, uncovered, for 45–60 minutes, stirring often.
Check consistency: the chutney should cling to a spoon without excess liquid. If it seems loose, keep simmering until thickened.
For a smoother chutney, pulse with a stick blender, or leave chunky.
Ladle into sterilised jars, seal, and cool. For best flavour, mature 2+ weeks before using.