Crisp blueberry oat slice

Ingredients

• 1 1/2 cup oats (or buckwheat flakes if gluten-free)
• 1 1/4 cup plain flour (or use gluten-free flour)
• 2/3 cup brown sugar, loosely packed
• 1/4 teaspoon salt
• 125 gram butter, melted
• 1 1/2 cup frozen blueberries
• 1 tablespoon cornflour
• 1 tablespoon lemon juice
• 2 tablespoon caster sugar
• icing sugar, to dust