Preheat oven to 375° and butter an 8x8” or 9x9” baking dish.
For the filling, mix the chopped rhubarb with the sugar, flour and nutmeg in a large bowl.
In a separate bowl, whisk together eggs and cream, then pour over the rhubarb mixture and stir to thoroughly combine. Pour into prepared baking dish.
For the topping, stir together flour, brown sugar, oats, baking powder, cinnamon and salt in a large bowl.
Add pieces of butter and work into the mixture with your hands, a fork or a pastry cutter until large crumbs form. Pour crumble topping mixture over the custard filling mixture.
Bake 30–35 minutes, until custard is set, topping is starting to brown and juices are bubbling.