Blend onion, ginger, garlic and green chillies into a paste and set aside. Now take a large mixing bowl, grate the boiled eggs and add two teaspoons of the paste, red chilli powder, coriander powder, garam masala powder, turmeric powder, cumin powder, salt, chaat masala, coriander leaves and cornflour. Combine it well and make small balls from the mixture. Now heat butter on a pan and shallow fry the kebabs till they turn golden brown in colour. Garnish it with onion slices and lemon wedges and serve.