1 tablespoon pickling spice, tied in a cheesecloth bundle
1 small bay leaf
1/2 medium green bell pepper, thinly sliced
1 medium sweet onion, thinly sliced
In a colander set in the sink, toss the green tomatoes with the salt and let stand at room temperature until almost all of their liquid has been released, at least 2 hours or overnight. Drain and rinse the tomato slices, then pat dry.
In a large saucepan, combine the vinegar, water, granulated sugar, brown sugar, pickling spice bundle and bay leaf and bring to a boil. Reduce the heat to moderate and simmer until reduced by one-third, about 30 minutes.
Add the sliced green tomatoes, green bell pepper and onion and cook them over moderate heat until the liquid is reduced to a thick syrup, about 2 hours. Discard the pickling spice bundle and the bay leaf.
Pour the green-tomato mixture into 2 hot sterilized 1-pint jars, stopping 1/4 inch from the tops. Wipe the glass rims and seal the jars. Let cool and refrigerate. Alternatively, process the pickle: Set the jars in a water bath, bring to a boil and boil for 15 minutes. Remove the jars from the water bath and let cool completely.
The unprocessed pickle can be refrigerated for up to 3 months. The processed pickle can be stored at room temperature for up to 1 year.