Heat up remaining milk, add sugar, stir till it dissolves. Don't boil it. Once it's close to boiling remove from heat and add potato starch dissolved in 1/2 cup of milk, stir vigorously. Let it cool down.
Once it's cool add pistachio paste and lemon juice, mix well, transfer to ice cream maker.
Usually it takes about 20 minutes till ice cream is done. Freeze about 15 minutes and serve.
Homemade ice cream can be stored up to 1 week in a freezer.