In a heavy-based saucepan, add the milk and a generous pinch of salt. Slowly heat the milk until it reaches 93C/199F.
Pour in the lemon juice or vinegar and stir. Remove the pan from the heat and continue to stir for 2-3 minutes.
Leave the milk to cool down in the pan for 15-20 minutes, or until the curds and whey have separated.
Using a fine-mesh sieve, gently strain the contents of the pan into a non-reactive bowl and leave to drain. Drain for 30 minutes for a very wet, soft ricotta, or overnight for a firmer ricotta. Save the leftover whey for another recipe.
Store in the fridge, in an airtight container, for 2-3 days.