I found this to be VERY good, even though I dislike sweet pickles. I liked especially the Vidalia onion prepared this way. The recipe comes from a cookbook by Susanna Hoffman, The Olive and the Caper: Adventures in Greek Cooking. At least three people in my family have copies of this cookbook!
3/4 lb zucchini
1/2 med onion (I used Vidalia)
1/2 tsp kosher salt (I used sea salt)
1 tsp dry mustard powder
1 tsp mustard seeds
1/2 tsp ground turmeric
1/8 tsp whole cloves
1/4 c dark brown sugar
1/3 c white wine vinegar
Slice zucchini into 1/4" rounds. Quarter onion and slice 1/8" thick. Put both in a colander, sprinkle with the salt, and toss. Let the zucchini begin to sweat, 10 to 15 min.
Mix everything else together in a saucepan. Bring to a rolling boil over high, then remove from heat.
Transfer the zucchini and onion to the pan, stir, and set aside to cool. When you can handle it, transfer the mix to a storage container and refrigerate overnight.