Peel the tomatoes: Score the bases, blanch 30–45 s in boiling water, shock in cold water, slip off skins. Chop, catching all juices. If peeling “feels optional,” enjoy fishing papery flecks from your teeth later.
Start the pot: Put onions, half the vinegar (225 ml), and the salt into the heavy pot. Cook on medium-low, lid ajar, 8–12 min until translucent—not brown. Browning will hijack the flavor; you’re making relish, not onion jam.
Add the rest: Tip in tomatoes (with juices), remaining vinegar, sugar, mustard seeds, cloves, paprika, and cayenne. If you’re nervous about spices floating around, tie the cloves in a tiny muslin bag but keep mustard seeds loose for proper “pop.”
Simmer and reduce: Bring to a gentle simmer. Skim foam, then cook uncovered 60–90 min, stirring more often as it thickens. You’re done when a spoon dragged across the pot leaves a track that slowly fills and the relish coats the back of the spoon.
Final seasoning: Taste at the end when acidity and sweetness have concentrated. Adjust salt. If it’s too sharp, give it five more minutes; reduction beats dumping in more sugar. If it’s too sweet, add a splash of vinegar and simmer 2–3 min.
Jar it Ladle hot relish into hot, sterilized jars, leaving ~5 mm headspace. Wipe rims, apply lids, and let cool undisturbed. Store in the fridge. For pantry storage, follow a tested local water-bath process; if you have to guess, don’t.
Mature: Leave it 48 hours before opening. Freshly cooked relish tastes shouty; time smooths it out.