I rarely use an egg to bind rissoles, it just isn't needed if you can manage to get the right texture when blending.
I just use a mix of meat, onions and bread in 2:1:1 proportions, I tend to put them into the food processor individually until grains are quite small but not baby mush. Then combine - with salt, pepper, worcester sauce and other spices for a final mix with just enough stock to get a stiff mix.
You could use lentils instead of the meat of course but you'd need lots of spices to make anything which wasn't very bland.
Form into patties and put in fridge for half an hour to make them a little firmer if you like - but not usually needed.
I usually fry in a pan with a bare sprinkling of oil but it isn't really needed.
p.s. as an afterthought, why not substitute some of the lentils with celery, kale or other tasty veg ground up? Tomato paste instead of the stock might add a bit of flavour.