Soak the flour: Mill 7.5 ounces soft white wheat berries, freshly milled (1¾ cups / 210g) fresh and combine with 6.4 ounces whole milk buttermilk or yogurt (¾ cup / 180g) in a medium bowl. Stir until a shaggy mixture forms. Cover and let soak at room temperature for 30–60 minutes (or overnight in the fridge). This softens the bran and creates a more tender, light crumb.
Macerate the fruit: Toss 8 ounces fresh strawberries, hulled & halved (1½ cups / 225g) and 6.4 ounces fresh rhubarb, sliced ½-inch thick (1½ cups / 180g) with 1.4 ounces cane sugar, for fruit (3 tbsp / 38g) and 0.1 ounces lemon zest (1 tsp / 2g) in a bowl. Let sit while the flour soaks to draw out the juices and develop a jammy quality.
Make the crumble topping: In a small bowl, mix 2.1 ounces freshly milled soft white wheat, for crumble (½ cup / 60g), 1.6 ounces rolled oats (½ cup / 45g), 2.3 ounces brown sugar or coconut sugar, for crumble (⅓ cup / 65g), 0 ounces cinnamon, for crumble (½ tsp / 1g), and 1 pinch pinch of salt, for crumble. Add 2 ounces cold unsalted butter, cubed, for crumble (4 tbsp / 57g) and use your fingers to rub everything together until clumpy and crumbly. Refrigerate until needed.
Preheat oven & skillet: Preheat oven to 350°F / 175°C. Place a 10-inch cast iron skillet in the oven for 10 minutes. to preheat. This gives the cake a beautifully crisp edge.
Make the cake batter: In a large bowl, beat 4 ounces unsalted butter, softened (½ cup / 113g) and 5.4 ounces cane sugar (¾ cup / 150g) until pale and fluffy, about 3–4 minutes. Add 2 eggs, room temperature (2 large / ~100g) one at a time, beating well. Mix in 0.2 ounces vanilla extract (1½ tsp / 6g). Add 0.2 ounces baking powder (1½ tsp / 6g), 0 ounces baking soda (¼ tsp / 1g), 0.1 ounces fine sea salt (½ tsp / 3g), and 0 ounces ground cardamom (½ tsp / 1g) directly to the soaked flour mixture and stir to combine. Gradually fold the flour mixture into the butter mixture until just combined — do not overmix.
Assemble in the skillet: Carefully remove the hot skillet from the oven and grease generously with butter. Pour in the batter and spread evenly. Scatter the macerated fruit evenly over the top, pressing lightly into the batter. Sprinkle the crumble topping generously over everything.
Bake: Bake at 350°F / 175°C for 38–42 minutes, until the crumble is golden, the edges pull away from the skillet, and a toothpick inserted in the center comes out clean. If the crumble browns too fast, tent loosely with foil after 25 minutes.
Cool & serve: Transfer the skillet to a cooling rack and rest for at least 10–15 minutes
