Prepare the Tomatoes: Bring a large pot of water to a boil. Score a small 'X' on the bottom of each tomato and blanch them for 30-60 seconds, until the skins split.
Transfer to an ice bath to cool.
Once cool, peel the skins, then halve or quarter the tomatoes and cut them into thick slices.
Combine : Peel and finely chop the onions.
In a large, non-reactive pot or enameled kettle, combine the sliced tomatoes, chopped onions, raisins, prunes, cider vinegar, brown sugar, ginger, cayenne, salt, cloves, coriander, and mustard seeds.
Simmer to Perfection: Bring the mixture to a boil over medium-high heat, then reduce the heat to a gentle simmer.
Cook, stirring occasionally, until the chutney has thickened considerably and has a jam-like consistency. This may take over an hour. To check if it's ready, drag a spoon across the bottom of the pot; it should leave a clean trail for a moment.
Jar and Seal: Carefully spoon the hot chutney into hot, sterilized jars, leaving at least ¼ inch / 0. 5 cm of headspace at the top. Wipe the rims clean, apply the lids, and seal tightly. For best results, follow proper canning procedures if you plan to store it long-term.
Adjust the heat to your liking. You can reduce the cayenne for a milder chutney or add a fresh, chopped chili along with the other for an extra kick.