Heat ghee in a pan. Reserve 3 tsp chopped onions for garnish and fry the remaining till light brown. Add green chillis, ginger and tomatoes and saute over medium heat for 2-3 minutes. Reduce heat, then add the red chilli powder, mixed vegetables and potatoes.
With a metal spatula, simultaneously mash and stir for 5 minutes. While stirring, keep sprinkling the ginger garlic paste, dissolved in 6 tablespoons water. Add salt and two thirds of butter. Increase the heat and stir well.
Sprinkle garam masala, coriander leaves, lemon juice and remove from heat.
Garnish with remaining chopped onions.
Apply the remaining butter on the bun-halves, place them on a pre-heated frying pan. Fry until golden brown on both sides. Serve with the bhaji and a lemon wedge