Put a deep-bottomed and non-stick pan on medium flame and add milk in it. Bring the milk to a boil and then add grated bottle guard. Stir to mix well and let it simmer for about 10-15 minutes.
Now, add sugar in the pan and give a nice stir, cook well till the milk is absorbed by the bottle gourd and thickens in consistency. add khoya in the pan along with ghee, and cardamom powder. Stir to mix once again.
Take a large plate and grease with a little ghee. Once the milk is absorbed completely, turn off the burner and transfer the prepared halwa into the plate. Spread it evenly and garnish with chopped pistachio and almonds. Keep it aside to cool at room temperature.
Once the halwa is cool enough, refrigerate it for 4-5 hours to set the burfi. Take out after some time and cut into desired shapes and sizes. Serve immediately.
