Mash bananas until smooth, stir in lemon juice.

In a small saucepan, bring water and sugar to a boil over high heat; reduce heat to medium and simmer for 2 minutes, until slightly thickened. Let cool to room temperature. (to speed up cooling of syrup place in fridge for 40 - 45 minutes.).

Stir cooled syrop into banana puree.

Freeze in ice cream maker according to manufacturers directions.

OR.

Place in a shallow pan covered with plastic wrap for at least 4 hours.