In a medium pot on medium heat, bring water and raisins to a light simmer. Reduce heat and cook 30 minutes. Strain the liquid into another container and set aside. Discard raisins (or use for another project).
In a heavy-bottomed pan, cook sugar on high, stirring occasionally until golden-amber in color. Working quickly but carefully, whisk raisin water into sugar (the mixture will steam and foam). Set aside.
Ice Cream Base:
Place milk and caramelized onions in a blender on high until it is a fine puree.
Place egg yolks into a bowl and set aside.
Heat onion milk, sugar and condensed milk in a pot on medium heat and stir occasionally until it comes to a light boil.
Slowly whisk a third of the hot milk mixture into egg yolks, then add to the pot. Whisk constantly until mixture reaches 160°F.
Remove from heat and pour into the cool milk to stop the cooking process.
Strain and set aside to cool completely.
Churn ice cream base as per the instructions on your ice cream maker. Serve with raisin caramel and top with optional sea salt.