Thaw the frozen raspberries in a bowl of cold water for about 10 minutes. Drain and pat dry with a paper towel. In a mixing bowl, combine raspberries with sugar, flour, cornstarch, and lemon juice, gently folding to coat the berries.

Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper.

In a mixing bowl, combine oats, flour, brown sugar, baking powder, and sea salt. Melt the butter and mix it into the dry ingredients until clumpy, reserving 1/2 cup for topping.

Press the remaining streusel mixture into the bottom of the baking pan. Bake for 10-12 minutes until slightly set.

Pour the raspberry filling over the crust.

Sprinkle the reserved streusel mixture over the raspberry filling. Bake for an additional 25-30 minutes until golden brown.

Cool at room temperature for 30 minutes, then refrigerate for at least 2 hours before slicing.