Special equipment: a 23-by-13-by-7-cm metal loaf pan
Chill the loaf pan in the refrigerator for at least 30 minutes.
Puree the avocado, milk, lemon juice and salt in a blender until smooth, about 1 minute; transfer to a large bowl.
Whip the cream in another large bowl with an electric mixer on medium-high speed until firm peaks form, about 2 minutes. With a rubber spatula, fold about 1 cup of the whipped cream into the avocado puree until combined. Fold the lightened mixture back into the whipped cream until well blended. Pour into the chilled loaf pan, and place in the freezer until firm, 4 hours or overnight.