Start by blanching the nuts. Take almonds and pistachios in a bowl and add boiling hot water to it. Let it sit for 20 minutes. Then discard the water. Peel the skin and slice them. Keep it aside.
Take milk in a wide, heavy bottom pan or kadai and turn the heat on medium.
Once the milk starts boiling, lower the heat to medium-low. It will start to form a thin layer of malai/cream. Now using spoon or spatula, collect that.
And stick it to the side of the pan. Within some time, it forms again and similarly stick to the sides. Continue doing the same till you reach ⅓ of its original quantity.
Few things to keep in mind: You have to stir it occasionally to make sure that it is not sticking to the bottom. But do not stir that often or quickly otherwise the cream layer will not form on top rather it will disintegrate into the milk.
Now add sugar and saffron. Mix and continue cooking and collecting the cream for another 5-7 minutes.
Now start scraping the sides and add to the thickened milk.
Mix gently, we do not want to break the large layers (aka malai lachha) and simmer for 1-2 minutes.
Turn off the stove. Add sliced almonds and pistachios. Mix and lachha rabdi is ready.
Use heavy bottom and wide mouth pan (not deep like a saucepan). Wide pan helps the milk to evaporate quickly and heavy bottom will prevent the milk from burning and scorching at the bottom of the pan.
Cook on medium-low heat throughout the rabri making process. Doing to slowly makes a big difference vs boiling and reducing it vigorously.
The cooking time given is approximate. It may vary depending on the shape, size of the pan and intensity of the flame.