This is the basics of the nut free, vegan recipe sheet that I followed (as far as I can remember).
The recipe used a precooked orange lentil base. The base could be substituted with badly mashed up tinned chickpeas, plain tinned haricot beans, butter beans or kidney beans. The recipe used a ratio of generally 2 parts base to 1 part filler but was very rough and didn't list amounts or cooking times. The burgers are shaped (not too thinly) then always quickly pre-fried on the hob in a frying pan prior to putting on the bbq. They work okay when pre-baked on trays in the oven too.
The lentils had to be carefully boiled dry after cooking so they weren't too wet - this was the different part. It was easier using the drained, tinned beans as a base.
Some of the burger recipes called for grated onions and some for sliced onions. I think it was the more 'solid' burgers that had the sliced onions. The mexican burgers that always fell apart used grated onion.
The filler Ingredients added to the base could be minced garlic, grated or sliced onion, smashed peas, grated carrot, grated courgettes, grated parsnips, grated potatoes (the grated vegetable step was the important part which helped keep the burgers together and moist whilst they were cooking). I've found using a spiralizer instead of a grater makes the burgers less prone to falling apart.
Seasoning like tomato puree, ginger, soya sauce, chilli paste and herbs were used. The only burgers I did that always fell apart were the mexican ones with diced pepper, sliced chillies, sweetcorn, sliced onion rings, grated courgettes and a red kidney bean base.
I found adding sweetcorn, diced carrot or diced mushrooms to generally results in burgers that fall apart. Grated carrot or pre-sauteed minced mushrooms works better.
The recipe suggested adding dry soup powder if the burgers were not holding together very well or as additional flavourings.