Combine the lactose free milk and sugar in a medium, heavy bottom saucepan. Place it over a low heat. I'd recommend using the smallest burner you have - I have found that is the easiest way to ensure you don't end up with burnt or curdled condensed milk.
Cook over this low heat, stirring intermittently, for at least an hour. During this time, it will gradually turn a blue grey colour, before eventually turning a golden brown, as per the photo.
If at any time the milk is burning to the bottom of the pan, or the mixture is bubbling wildly, lower the heat. Slow and steady!
The condensed milk is ready when it can lightly coat the back of a spoon, and is still thin but more viscous than at the beginning. It will thicken considerably once cooled.
Store in an airtight container in the fridge, and use for anything from no churn ice cream to caramel slices to Vietnamese iced coffees.