1-Preheat the oven: Preheat the oven to 350 degrees Fahrenheit. Grease a 9 by 9-inch baking dish and line it with parchment paper, then grease the parchment as well.

2-In a mixing bowl: whisk together the flour, baking soda, and salt.

3-Add the oats, brown sugar, and granulated sugar, blending them while breaking up any small clumps of brown sugar with your fingertips.

4-Whisk the vanilla into the melted butter and pour it over the oat mixture. Stir with a spatula until evenly moistened.

5-Press two-thirds of the crumble mixture into the bottom of the prepared baking dish. Set aside the remaining one-third for the topping.

6-In a medium bowl, toss the diced strawberries and rhubarb with the lemon juice.

7-In a small bowl, whisk together the 1/3 cup granulated sugar and cornstarch, then pour it over the fruit mixture and toss to coat evenly.

8-Spread the fruit mixture over the bottom crumb layer in the baking dish.

9-Sprinkle the remaining crumble mixture evenly on top.

10-Bake for 45 to 50 minutes, until the top is golden brown and crisp and the filling is bubbling.

11-Cool on a wire rack until warm, then cut into squares and serve. For an extra treat, serve warm with vanilla ice cream.