Preheat the oven to 450°F and prepare one or two baking sheets with cooking spray
Put the flour into a bag and season with salt, pepper and cayenne.
Pour the buttermilk into a large bowl and pour half the panko crumbs onto a plate.
Set the baking sheet within easy reach.
Working in batches, shake the onions or other veg with the flour, shake off the excess before dipping into the buttermilk batter then coating with bread crumbs. Arrange on the baking sheet.
Dividing the bread crumbs into two portions helps prevent them from clumping up as you work. Add more as needed.
When the tray is full spray the tops with more cooking spray and bake for about 20 minutes or until the coating is brown and crispy and the veggies have softened. Check after 10 minutes and turn the veggies and the tray as necessary for even cooking.
Serve with your favorite sauces. We liked the sriracha mayo with the butternut squash and the BBQ sauce with the onion rings. The eggplant was great with the teriyaki.
To make the sriracha mayo mix a teaspoon or so into a half cup of mayo. Add more if you like it hotter.
Leftovers, if you have any, can be reheated/recrisped in the oven.