In a large saucepan (preferably stainless steel) combine green tomatoes, onions, green and red peppers with the pickling salt. Cover and let sit for 12 hours (overnight works too) in a cool, dark place.
On Canning/Preserving Day
Drain and rinse vegetables, pressing out any excess moisture; set aside. Prepare your jars and lids (6x250ml jars). Note: I sterilize my jars in the oven at 225 degrees Fahrenheit and I keep my lids in a saucepan on simmer.
In a large saucepan combine vinegar, garlic, brown sugar, mustard, salt, and ginger. Tie clove, celery seed and cinnamon in a square of cheesecloth, creating a spice bag. Add this spice bag to your saucepan. Mix well and bring to a boil.
Add drained tomatoes, onions and peppers and simmer for 1 hour, stirring frequently, until tomatoes appear transparent. Discard spice bag.
Ladle in the hot relish into prepared jars leaving 1/2 inch (1 cm) of headspace. Using something non-metal (I use a chopstick), remove any air bubbles. Wipe the rim of the jar and place lid on the jar. Apply screw band and tighten until resistance is met.
Once all your jars are full place them in the canner. Cover the canner with water and bring to a boil. For 250ml jars they will be in the canner for 10 minutes (15 minutes for 500ml jars). It’s important to know the altitude where you live as canning times will vary. Please see the links I provide below.
After the appropriate amount of time, remove jars from the canner and place somewhere where they can sit for 24 hours undisturbed. After they cool check the seal (sealed jars will curve downwards). Remove the screw bands, wipe down your jars and store. I store my jars without the screwbands and just put one one when I open something to use.