Heat the olive oil in a large frying pan over medium-high heat. Add the diced onion and cook for 2-3 minutes until softened. Stir in the garlic, ginger, cumin, garam masala, coriander, cardamom, and turmeric. Cook for 1 minute until fragrant.
Add the cauliflower florets and drained chickpeas to the pan. Stir well to coat everything in the spices.
Reduce the heat to medium-low. Pour in ¼ cup of water, cover with a lid, and cook for 10-15 minutes, stirring every 5 minutes to prevent sticking. The cauliflower should be soft and break apart easily when pressed with a spoon. Remove from the heat and allow to cool slightly.
Transfer the mixture to a food processor along with the ground flaxseed and a pinch of salt. Pulse until mostly smooth. The mixture should come together into a soft paste, but a few small chunks are fine. Transfer to a mixing bowl.
Stir the plain flour into the mixture. It will still be quite soft and sticky – this is normal. Cover and place in the fridge for at least 30 minutes or until no heat remains in the mixture.
Once chilled, divide the mixture into 4 portions. With floured hands, shape each portion into a patty about 2 cm thick.
Heat a little oil in a frying pan over medium heat. Cook the patties for about 5 minutes on each side until golden brown and cooked through. Reduce the heat if needed to stop them from burning before the centre is cooked through.
In a small bowl, mix the vegan mayonnaise with the green curry paste to taste. Lightly toast the burger buns. Spread the avocado over the base of each bun, add salad leaves, then top with a patty and a generous dollop of the green curry mayo. Finish with the top bun and serve.