Preheat your oven to 375°F (190°C) and adjust the rack to the center position. Lightly grease your baking dish.

In a large mixing bowl, combine the sliced fresh rhubarb, chopped strawberries, and lemon juice. Toss until coated, then add granulated sugar and cornstarch, mixing thoroughly.

Transfer the fruit mixture into the greased baking dish and spread evenly across the bottom.

In a separate bowl, combine brown sugar, flour, oats, cinnamon, and salt. Cut in softened butter and vegetable oil until the mixture resembles large crumbs.

Sprinkle the topping evenly over the fruit filling.

Bake for 45-50 minutes until the topping is golden brown and the filling is bubbling.

Remove from the oven and cool for about 15 minutes on a wire rack to set before serving.

Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream.