Methods

1. Preheat the oven to 350°F. Line a 9x13 pan with parchment paper and set aside.
2. To make the crust, start by browning the butter. Melt the butter in a medium saucepan over medium heat. Cook the butter, stirring often, until it foams and then browns, about 4-6 minutes. Transfer to a large mixing bowl, reserving 2 tablespoons of browned butter in a separate small bowl and set aside.
3. Combine the rolled oats, all-purpose flour, white and brown sugars, baking powder and salt in a food processor and pulse until the oats are slightly broken down and the dry ingredients are the texture of very coarse sand, about 10 pulses. Transfer the oat mixture to the large mixing bowl with the browned butter and gently stir until a sandy, crumble forms.
4. Transfer 2/3 of the oat crumble mixture to the parchment-lined baking pan, spreading it out into an even layer and gently pressing it down to form a crust. Refrigerate for 20 minutes.
5. For the crumble, add the reserved two tablespoons of browned butter and chopped pistachios to the remaining oat mixture. Stir to combine and set aside.
6. While the crust is chilling, prepare the filling. Combine the strawberries, 1/4 cup of sugar, lemon zest, lemon juice, cardamom, vanilla and salt in a medium bowl and toss to combine. Let macerate for 20-30 minutes. Taste the fruit and adjust sugar, as needed. Add the all-purpose flour and cornstarch and stir to coat the fruit. Set aside.
7. Remove the crust from the refrigerator and bake until very lightly golden, about 10 minutes.
8. Give the strawberry mixture a stir and then pour onto the crust, gently spreading the fruit into an even layer. Top with the reserved oat and nut crumble mixture. If you want larger crumbles, squeeze the mixture together in your hands before sprinkling it over.
9. Bake the bars until the fruit is cooked through and jammy and the crumble is deeply golden brown, about 45-50 minutes. Remove from the oven and cool to room temperature. Refrigerate any leftover bars (the fresh fruit has a lot of moisture and no one wants moldy crumble bars!) and enjoy within 4-5 days.