Green Tomato Piccalilli

Combine the chopped green tomatoes and onion in a large non-reactive bowl, sprinkle with kosher salt, and stir to coat evenly. Cover the bowl and let the mixture sit at room temperature for 4 hours to draw out excess moisture.
Drain the tomato and onion mixture in a colander and rinse thoroughly under cold running water to remove the salt.
In a large Dutch oven or heavy-bottomed pot, combine the white vinegar, sugar, mustard seeds, celery seeds, turmeric, and red pepper flakes. Bring the mixture to a boil over medium-high heat, stirring occasionally until the sugar dissolves completely.
Whisk together the all-purpose flour and cold water in a small bowl until a smooth, lump-free paste forms.
Reduce the heat to medium-low and gradually whisk the flour paste into the simmering vinegar mixture until fully incorporated and the liquid begins to thicken into a smooth, gravy-like consistency.
Add the drained tomato and onion mixture to the pot and simmer gently for 15 minutes, stirring occasionally, until the vegetables have softened slightly but still retain some texture.
Ladle the hot piccalilli into clean, sterilized pint jars, leaving 1/2-inch of headspace, and seal immediately according to proper canning procedures for long-term storage.