Cut the cooked beetroot into thick slices, and set aside.
Toast the cashew nuts for a few minutes in a dry frying pan over a medium heat. Be careful, as they can catch and burn quickly, so stir them constantly and watch them carefully.
Add the cashew nuts to a food processor, along with the sage, red onion, and a generous pinch of salt and pepper. Blitz to form a rough paste, and transfer to a bowl.
Put the flour in one small dish, the beaten eggs in another, and the breadcrumbs in a third dish.
One at a time, coat each slice of beetroot in the flour, then the egg, then the sage and onion mixture, and then the breadcrumbs. It can get a bit messy, so I find it easiest to use one hand to coat the beetroot in the flour and then drop it into the egg, and the other hand to coat it in the egg, onion mixture, and breadcrumbs. Place the coated beetroot slices onto a plate.
Drizzle a little oil into the Actifry pan (with the rotating paddle removed), and add the beetroot nuggets in a single layer - you'll probably need to cook them in 2 batches. Drizzle a little more oil over the top, and cook for 15 minutes until golden brown and crispy.