Easy blueberry rhubarb crisp with a marzipan oat crumble makes the best summertime dessert to feed a crowd. Make sure to bake the fruit filling first so that it has time to cook and gel properly. The combination of blueberry and rhubarb is also referred to as bluebarb or rhuberry.
Preheat the oven to 375ºF (190 °C). Butter the inside of a large 10 inch baking dish that is at least 3 inches deep.
Prepare the blueberry rhubarb filling
In a large bowl, combine the sugars with the fine kosher salt. If you are using a thickener, mix it with the sugars to make sure it is evenly dispersed.
Add the fruit to the bowl and toss everything together to coat evenly. Transfer to the buttered baking dish, cover with foil, and bake on the middle rack for 45 minutes, stirring after 30 minutes.
Prepare the crumble topping
In a large bowl, stir together the grated marzipan, flour, sugar, ground almonds, rolled oats and salt. Add the melted butter and mix to form a crumble.
When the fruit has baked for 45 minutes, take the dish out of the oven, give the fruit a gentle stir, then sprinkle the crumble topping over the fruit.
Put the baking dish back in the oven and bake the crumble, uncovered, until it is golden brown and the filling is bubbly, about 30 more minutes.
Let rest before serving.
Notes
You may want to set the baking dish on a large cookie sheet or place a cookie sheet on the rack below to catch the bubbling hot fruit juice drips as they may burn on the bottom of your oven.