Heat water in a non stick pan. Add alum and green chillies and cook for 2-3 minutes. Strain the chillies. Repeat this process 3-4 times.
Drain and grind the chillies coarsely. Heat ghee in a non stick pan, add semolina and sauté for a minute.
Add the ground chillies and sauté for 3-4 minutes. Add cardamom powder and mix well.
Add khoya and sugar and cook on a low heat till the halwa begins to leave the sides of the pan. Transfer the halwa into serving bowls, garnish with chopped nuts and serve.