• Preheat the oven to 350°F (180°C).

Filling:

• Rinse the rhubarb and cut it into ½-inch slices. If using fresh strawberries, hull them and cut them in half.
• In a large bowl, combine the chopped rhubarb and mixed berries, granulated sugar, cornstarch, lemon zest, lemon juice, vanilla, and a pinch of salt. Toss until the fruit is evenly coated, then transfer to a baking dish.

Topping:

• In a separate bowl, combine the flour, sugar, and salt. Add the cold butter and rub it into the flour mixture with your fingertips until the mixture resembles coarse crumbs. 
• Sprinkle the crumble evenly over the fruit, pressing it around the edges so the filling is completely covered. Scatter the sliced almonds on top.
• Bake for 40-50 minutes, or until the filling is bubbling around the edges and the topping is golden.
• Let the crumble rest for all least 10-15 minutes before serving. Enjoy warm on its own or with vanilla custard or vanilla ice cream.