Place all of the rhubarb, sugar, water, and salt in a medium-sized saucepan and place over medium-low heat. Leave the rhubarb to cook until it completely falls apart.
Take the rhubarb off of the heat and transfer to a bowl. Let cool to room temperature before covering and transferring to the fridge.
In a large bowl, whisk to combine the flour, oats, sugar, almonds, and salt. Stir in the melted butter until well-integrated.
Spread the finished mixture evenly over the surface of the baking sheet. Bake the crisp for 20-25 minutes. Let cool. Break it into chunks.
While the milk is heating, place the egg yolks, sugar and salt in a medium-sized bowl and whisk to combine. Temper the eggs and gently heat the ice cream base
Churn the ice cream base. Sprinkle in the crisp. Transfer half of the ice cream to the chilled pan. Spoon half of the rhubarb over top and swirl the two together. Repeat and freeze for 4 hours.