Wash and dry the tomatoes. Trim the ends of the tomatoes and onions, and peel the onions. Dice into ¼-inch (6 mm) pieces and place in a large glass or ceramic bowl.
Sprinkle the pickling salt over the tomatoes and onions, then stir well to combine. Cover the bowl with a clean tea towel or a reusable shower cap, and let it rest at room temperature for 8 hours or overnight.
Drain the tomatoes and onions well through a sieve, pressing gently to remove excess liquid. No need to rinse.
Place the drained vegetables in a large pot. Add the vinegar, sugar, cinnamon, allspice, dry mustard, cloves, and nutmeg. Stir to combine, then bring to a gentle simmer over medium heat.
Once the liquid in the pot is bubbling, continue cooking gently for 20 to 25 minutes, reducing the heat if needed, until the liquid is reduced and clings to the vegetables.
Ladle the hot chow into clean jars. Let cool to room temperature, then cover with lids and refrigerate.
Notes
Use firm, fully green tomatoes without blemishes or signs of ripening for the best texture.
Make sure the bowl you use for salting is glass or ceramic, as metal can react with vinegar and salt.
Keep the heat at a gentle simmer; boiling too hard can break down the vegetables too much.
Storage
Refrigerate the chow in clean jars for up to 1 month.
If you prefer a longer shelf life, process the jars of hot chow in a boiling water bath for 10 minutes and store sealed jars in a cool, dark place for up to 1 year.