Preheat oven to 375°F (190°C). Lightly butter an 8x8-inch baking dish or similar sized pan.
In a large bowl, toss strawberries and rhubarb with granulated sugar, cornstarch, lemon juice, vanilla extract, and a pinch of salt. Let sit 5–10 minutes.
Mix oats, flour, brown sugar, cinnamon, and salt in a bowl. Cut in cold butter until mixture resembles coarse crumbs with some pea-sized pieces. Stir in nuts if using.
Pour the fruit into the prepared dish and spread evenly. Sprinkle the crumble topping over the fruit, pressing lightly in places.
Bake for 35–40 minutes until the topping is golden and the filling bubbles at the edges. Tent with foil if the top browns too quickly.
Let rest 10–15 minutes before serving. Serve warm with vanilla ice cream or whipped cream.
Notes
If using frozen fruit, add an extra tablespoon of cornstarch to the filling and increase baking time by 5–10 minutes.
To make gluten-free, use 1:1 gluten-free flour and certified gluten-free oats.
Assemble ahead and refrigerate for up to 24 hours before baking, or bake and reheat when needed.