Preheat your oven to 160ºC (fan-forced) or 170ºC (conventional).

In a medium saucepan, combine the frozen berries and caster sugar. Cook over medium heat for about 10-15 minutes, until the berries break down and the sugar dissolves. Mix the corn starch with the lemon juice and add it to the berry mixture. Stir and bring to a simmer for 1 minute. Remove from heat and cool in the fridge or freezer.

In a medium mixing bowl, combine the flour, brown sugar, and rolled oats. Using a fork or your hands, cut the butter into the mixture until it resembles coarse breadcrumbs with chunky bits.

Line a 22 cm square baking tin with baking paper. Firmly press half of the crumble mixture into the bottom of the tin. Pour the cooled jam filling over the base and spread evenly. Crumble the remaining oat mixture over the top of the jam.

Bake in the preheated oven for 25-30 minutes, until the jam starts to bubble around the edges and the crumble topping is golden and crisp. Allow to cool in the fridge for about 30 minutes before cutting into 16 squares.