Prepare your ice cream maker, if using. This may require you to chill your maker in the freezer overnight. If you don't have an ice cream maker, use my guide on making ice cream without an ice cream maker.

Preheat the oven to 350 degrees. Line a baking sheet with parchment paper or a silicone baking mat.

Mix together the ingredients for the crumble in a bowl. Rub the butter into the flour and oats to form large crumbles.

Place the crumbles on the baking sheet and bake for 8 - 10 minutes. Allow to cool completely.

Place the yolks and half the sugar into a bowl and whisk together.

Make an ice bath that is heavy on the ice.

Add the remaining sugar, cream, milk, and salt together in a medium saucepan and mix. Heat until the mixture is steaming and gently simmering at the edges. Do not allow to boil.

Whisk the egg yolks vigorously as you slowly drizzle half the milk mixture into it. Begin to whisk the remaining milk mixture in the saucepan and drizzle the egg/milk mixture back into the saucepan.

Stir the mixture in the saucepan until it begins to thicken. It should coat the back of a spoon when dipped into it. Remove from heat. Stir in the vanilla, rosewater, if using, and rhubarb jam. Place the saucepan in the ice bath, being careful not to allow any water into the custard mixture.

Once the mixture is cool, transfer the custard into a gallon freezer bag. Place in the refrigerator until it is completely chilled - at least four hours, preferably overnight.

Place the ice cream in your ice cream maker. This recipe makes a lot, so you may have to do it in batches for the best results. Make the ice cream maker according to manufacturer instructions, drizzling in the creme fraiche as it churns. During the last minute of churning, add in the crumble. You may not use up all of it.

Pour the finished ice cream in air tight containers and freeze until solid - at least four hours.