Last summer we started having garden burgers and I've been making them since.
I put whatever veggies I have from the garden, or in winter, the fridge, and let them sit in salt for a while to soften. I like to use things like chopped peppers, zucchini, onions, mushrooms, whatever I have and feel like using.
I usually use 1/2 veggies, 1/2 hamburg.
Boy is this a delicious burger!