Cut up onions roughly and place in a bowl. Sprinkle with 2 teaspoons of salt (if you don’t have Trapani, use another semi-fine grained, natural salt) and toss thoroughly, cover and leave overnight. You can leave at room temperature as long as the onions are pressed down into their own juice.
Repeat step 1 for the tomatoes.
The next day, sterilise jars and lids (you need enough jars to hold approximately 2.5 litres) in a preheated oven at 150°C for 20 minutes with lids off.
Strain off the liquid into a bowl from both the tomatoes and onions. Set the liquid aside to bottle later with your chutney – it makes a delicious base for tomato soup as well as other soups.
Combine all the spices, sugar and stevia with just enough vinegar to bring them together to make a paste.
Place onions and tomatoes into a large, heavy-based stockpot with enough white wine vinegar to nearly cover them. Add a good dash of apple cider vinegar as well as the spice paste. Bring to the boil and gently boil for 40 minutes, stirring occasionally.
Fill the hot chutney into the sterilised jars, being careful not to spill it onto the opening of the jar. Leave space at the top of the jar so the chutney does not touch the lid.
Seal the jars tightly, then place in a preserving unit. Fill water to just above the jars, so they are submerged. Heat treat for 30 minutes at 90°C. (If you don’t have a preserving unit, you can also do this in small batches on the stovetop.) Place jars into a stockpot, fill with water, almost to the top of the jars, then bring to a very gentle boil. Boil for 30 minutes, being careful to keep the boil very gentle.