Grate zucchini, then sprinkle grated zucchini with a little salt. Set aside for 5 minutes, while you prep everything else.
Grate potatoes, then using your hands squeeze water out of the potatoes. The best way to do this is by taking a small handful at a time.
Squeeze water out of the grated zucchini.
Add potatoes, zucchini, gram flour, sliced onions, red chilli powder, ground coriander, ground cumin, ground turmeric, green chillies, amchur powder, chopped coriander into a bowl. Mix and massage together.
Add oil into a pan and heat over low heat.
Pack Bhaji mixture into a 1/4 cup, carefully take it out, shape into a very rough ball, and flatten. Carefully place into the pan. Add as many Bhajis as you can into your pan, then cover with a lid and cook for 3-4 minutes or until the bottom side is crispy and golden. Flip and cook the other side until golden. Repeat until you have no mixture left.
Cut buns in half, spread a little vegan yoghurt on the bottom half, swirl in a little Mint Coriander Chutney. Place Onion + Zucchini Bhajis over the sauce, then top with thinly sliced cucumbers, drizzle a little more Mint Coriander Chutney and a little more vegan yoghurt. Top with lettuce, then finally, close the burger.