Finely chop the onion in your food processor.
Add the olive oil to a large pot and warm over medium heat. Add the onions and cook slowly till translucent, but not browned.
While the onions are cooking slowly, quarter the tomatoes and blitz in the food processor until it is totally pulped. See note.
Add the garlic, ginger, black mustard seeds and chilli to the pot and cook till fragrant.
Add the tomato pulp, pineapple, sugar and red wine vinegar and season with salt and pepper.
Cook on medium to low heat, stirring often, for 60 to 70 minutes until it is thick and jammy. Add the sherry vinegar about 10 minutes before the end of the cooking time.
While the chutney is cooking, wash the jars and lids in warm soapy water and rinse them in boiling water from a kettle - I do this in the sink. Let the jars and lids dry upside down on a clean kitchen towel until needed.
Once the chutney is ready, spoon it into the jars while still hot. Fill the jars to the top and close tightly. Wipe down the jars with a damp cloth to clean off any spillage. Jars can be kept in a cool food cupboard for about 2 months. Store in the fridge once opened.