Heat the oven to 375°F (190°C). Butter the baking dish and place it on a rimmed sheet to catch any overflowing berry juice.
Inspect and dry the berries. Quarter large strawberries so they cook at a similar rate to the smaller fruit.
Mix fruit sugar and cornstarch before tossing with berries and lemon juice. Turn everything gently to coat without crushing the raspberries.
Spread fruit in the dish, scraping in the sugar mixture. Keep the depth even so the filling heats through at the center.
Combine topping flour, brown sugar, salt, and cinnamon. Cut in cold butter until you have uneven crumbs, squeezing a few into larger clusters.
Scatter crumbs over the fruit without compressing them. Bake 35–45 minutes, until golden and bubbling through the center gaps.
If only the edges are bubbling, continue baking and shield the topping if needed. The central filling must heat enough for cornstarch to thicken.
Rest at least 25 minutes before serving. Spoon into bowls rather than expecting firm slices; the warm filling should stay juicy but lightly thickened.
