Pre heat oven to 200c / 390 ff
Place a baking rack/cake cooling rack over a rimmed baking sheet (my secret trick for no flip!).
Slice onion in 1 cm / 1/3 inch rings - keeping any small centre pieces for another use (see notes).
Mix together flour, paprika, garlic powder & salt.
Place 1/4 cup of flour mixture in a medium sized bowl, add beer 1 - 2 tbsp at a time to avoid any lumps & mix until fully incorporated.
In another medium bowl, combine parmesan & breadcrumbs then add melted butter to crumbs, mixing well to coat entirely.
Take 2 - 3 onion rings at a time, and coat them in flour mixture (see notes).
Using a fork, transfer rings to bowl containing beer mixture, turning to coat (see notes).
Transfer rings back to flour mixture, turning to coat.
Using the fork, transfer rings back to beer mixture again, turning to coat.
Transfer rings to breadcrumb mixture, & using a clean fork turn gently to coat well & lightly pressing crumbs on if needed, repeat with remaining rings.
Place onion rings in a single layer on prepared cake rack/baking sheet.
Bake for 15 mins & serve immediately with Dijon mayonnaise.