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1
Dice all vegetables to a uniform 5 mm. Combine in a large non-reactive bowl, toss thoroughly with the salt, and leave uncovered for 4–8 hours or overnight at room temperature. The salt draws out the excess moisture and firms the cell walls. At the end of the salting period, drain the vegetables in a colander, rinse briefly under cold water, and press firmly to expel as much liquid as possible — the drained mass should feel almost dry to the hand.
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2
Sterilise the jars and lids in a boiling-water bath while the vegetables drain. Hold them submerged and hot until needed.
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3
Combine the cider vinegar, sugar, mustard seed, celery seed, turmeric, and mustard powder in a large stainless or enamelled pot. Bring to a full boil over medium-high heat, stirring until the sugar is dissolved and the brine runs clear and deeply yellow.
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4
Add the drained vegetables to the boiling brine. Stir to coat every piece. Cook at a steady simmer for 10–15 minutes — this is where the dish lives or dies: the vegetables must heat through completely and absorb the mustard-turmeric flavour without sacrificing their structure. At 10 minutes, press a piece of green tomato between thumb and finger; it should yield with resistance and spring back slightly, not collapse. Pull from the heat the moment that crispness is confirmed.
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5
Pack the hot relish immediately into the hot sterilised jars, pressing down to eliminate air pockets and leaving 1 cm headspace. Wipe the rims clean, seal, and process in a boiling-water bath for 10 minutes. Remove and leave undisturbed on a rack; the jars will ping as the lids seal. Any jar that does not seal must be refrigerated and used within two weeks.