Peel bananas and cut into about 1/4 inch thick rounds. Place in a resealable bag and freeze for about 2 hours or until frozen.
In blender, combine sweetened condensed milk (place this first to make it easier for the blender), bananas and lime juice. Process until smooth, occasionally scraping with a spatula to ensure even blending.
Transfer mixture into a loaf tin or a container with tight fitting lid and gently fold in walnuts. You can enjoy it at this point as soft serve. Otherwise, wrap container with a plastic film, pushing down plastic onto surface of ice cream (plastic film should be touching ice cream, this is to prevent ice crystals). Freeze for another 2 to 3 hours or until firm.