Preheat oven to 375°F (190°C). Lightly spray a 1½- to 2-quart baking dish with baking spray with flour.

In a large bowl, stir together berries, ⅔ cup (133 grams) sugar, cornstarch, lemon juice, and ½ teaspoon (1.5 grams) salt until combined, slightly mashing berries to release juices. Pour mixture into prepared pan.

In a medium bowl, whisk together flour, oats, 2 tablespoons (24 grams) sugar, baking powder, and remaining ½ teaspoon (1.5 grams) salt. Using a pastry blender or your hands, cut in cold butter until butter pieces are pea-size; stir in milk. Using a 3-tablespoon spring-loaded scoop, scoop dough, and drop onto berry mixture in pan. Using your hands, slightly flatten dough, if desired. Sprinkle remaining 2 tablespoons (24 grams) sugar onto dough.

Bake until topping is golden and berry mixture is thick and bubbly, 40 to 45 minutes, covering with foil during final 20 minutes of baking to prevent excess browning. Let cool on a wire rack for at least 30 minutes. Serve warm or at room temperature.