Heat the oven to 375°F (190°C). Butter the dish and set it on a rimmed sheet. Use rhubarb stalks only, discarding all leaves.
Wash, dry, and prepare the strawberries and rhubarb. Keep the stalk pieces fairly small so they soften alongside the berries.
Mix fruit sugar and cornstarch, then toss with fruit and vanilla. Spread into the dish, distributing both kinds of fruit evenly.
Mix topping flour, oats, brown sugar, and salt. Cut in the cold butter until the mixture looks crumbly with some larger clumps.
Scatter topping across the fruit loosely. Avoid pressing it down, which can prevent the inner crumbs from crisping.
Bake 40–50 minutes until topping is golden and the center filling bubbles. A rhubarb piece in the middle should pierce easily.
Shield the top with foil if it colors before the fruit softens. Continue until the center reaches the same bubbling stage as the edges.
Cool 25 minutes before serving so the juices thicken. Spoon into bowls and offer cream or ice cream separately if desired.
