Cut the zucchini and squash lengthwise into ⅛” slices, using a mandolin or sharp knife. You want to make ribbons.
Spread the veggies and onion in a single layer on 2 parchment paper lined baking sheets.
Sprinkle with salt. Let stand 1 hour. Transfer to colander, rinse and drain.
Place in wide-mouthed 1-qt. jar, filling to ½” from top.
Bring vinegars to boil in saucepan.
Add sugar and cook 3-5 minutes, stirring constantly or until dissolved.
Stir in black pepper, celery seeds, mustard seeds, dry mustard and Tabasco (if you want spicy.) Bring to a boil, immediately remove from heat and pour into jar over veggies.
Cool 1 hour.
Cover and chill 3 days before serving. Refrigerate up to 2 months.
Recipe by Geaux Ask Alice! at https://geauxaskalice.com/pickled-zucchini-squash-ribbons/