In a food processor, whizz up the peeled beetroot roughly.
Add all the rest of the ingredients and whizz up again until you have a burger-ish kind of mass. NB Don't over process! You want some texture.
Form into patties. How many you make depends on how big the patties are. I usually get 4 burgers out of this.
If the patties are looking a little wet, add more dry linseed into it to soak up the juices. This will help bind it further.
Refrigerate the patties for an hour (or freeze overnight) to allow them to firm up.
Pan fry or oven bake until crisped up on the outside and cooked through. (about 5 mins each side).
You need to form a 'crust' before flipping the burger to help these hold together, so be patient while the burgers get a little bit of a crust. Use a thin slice to flip the burgers. They are a little big breakable so you want to pan fry these on a flat surface or use a flat griddle on the BBQ. Alternatively oven baking works really well too. (Bake at 180℃/360℉ for around 15 minutes or until crispy)
Serve on a bun with ALL the burger things you love.
Notes
If you don't have a food processor, grate the beetroot.. Use washing up gloves to avoid staining your hands! Mince the onion really finely, then use a potato masher or fork to mash up the lentils to create the burger.