You need to start with the crumble topping first, because it needs to chill in the fridge, and then make the fruit filling.
Make the crumble: In a medium bowl, stir together the melted butter, flour, sugar, salt, vanilla, and baking powder. It won’t look like a crumble yet - don’t worry, it will turn out perfectly crunchy! Spread it on the sides of the bowl so that all the crumble can chill evenly.
1 ½ sticks (170g) butter, 1 ½ cups (190g) flour, ¾ cup (150g) sugar, ¼ teaspoon salt, 1 teaspoon vanilla extract, 1 teaspoon baking powder
Put the bowl with the crumble in the fridge while preparing the fruit filling.
Preheat the oven to 400°F (200°C).
Make the fruit filling: Wash the strawberries and rhubarb, then pat them dry with paper towels.
1 lb (450g) rhubarb, ¾ lb (340g) hulled strawberries
Slice the rhubarb into ⅓ to ½ inch (1 cm) pieces and cut larger strawberries in half or into quarters. Leave small and medium-sized strawberries whole — if you chop them too small, they will either turn to mush or make the crumble watery.
Add the fruit to a 9x13 inches (23x32 cm) baking dish, or similar.
Stir together the fruit with sugar, cornstarch, lemon zest, vanilla, and lemon juice.
½ cup (100g) sugar, 1 tablespoon cornstarch, zest of 1 lemon, 1 tablespoon pure vanilla extract, 1 tablespoon lemon juice
Take the bowl with the crumble out of the fridge, break it into smaller pieces with a fork.
Sprinkle the crumble over the fruit making sure you have a mixture of smaller and bigger chunks.
Sprinkle with 1 tablespoon of sugar.
Bake for about 35-40 minutes or until the crumble is nicely golden brown.
Enjoy!