Make the raspberry filling. Add to a saucepan the raspberries, orange zest, orange juice, sugar, and salt, and stir to combine. Transfer the pan to a burner set to medium-low heat and mash the berries with a fork, trying to break down any clumps.

Once the berries have completely broken down and the mixture begins to thicken, use a sieve to sift in the corn starch and quickly whisk it in. Once the mixture thickens and begins to bubble very slowly, whisk in the vanilla bean seeds or extract and remove from heat. You want the filling to look spreadable but not runny, and keep in mind it will thicken up slightly as it cools.

Preheat oven to 350°F. Grease a 9-inch square tin and line with parchment paper.

Make the crumble base and topping. In a food processor, blitz half of the oats into fine pieces.

Add in the flour, sugar, salt, and baking powder and pulse until combined.

Add in the butter pieces and process on low until the mixture starts to clump together like dough.

Add in the remaining oats. I found that it was easier to transfer the dough to a mixing bowl and toss it with my hands to incorporate the whole oats. This also helps you get the right shape and size for the topping.

Dump half of the dough into the lined tin and us your hands to press it evenly across the bottom. If there are gaps, fill them with more dough. To get a flat surface, use a measuring cup to tamp down the dough.

Spread the cooled raspberry filling evenly across the base using the back of a spoon or an offset spatula.

Crumble the remaining dough over the filling.

Bake on the middle rack of the oven for about 40 min or until topping is golden brown.