Cook the onion in a small saucepan with the olive oil until soft and translucent, about 8–10 minutes, then add the garlic and cook for 2 minutes more, until fragrant. Remove from the heat and set to one side.
Combine the grated beetroot, beans, mustard, spices, dill, oats/breadcrumbs and seasoning in a bowl. Use your hands to work the mix together until cohesive and easily shaped.
Shape the mix into burgers about 2cm thick and place them on a tray or plate. Put them into the fridge to firm up for an hour or so.
Heat a large non-stick frying pan with enough oil to cover the surface of the pan and about 1cm deep, and fry the burgers over a moderate heat for 2–3 minutes on each side, until crisp and the interior is hot.