How to Make It
Preheat oven to 425°F. Grease a 12-cup muffin pan with cooking spray. Line a baking sheet with parchment paper, and lightly coat parchment with cooking spray. Toss together strawberries, rhubarb, and 2 tablespoons of the sugar on prepared baking sheet; spread in an even layer, and bake in preheated oven until tender, about 20 minutes. Remove from oven, and set aside to cool completely.
Combine brown sugar, pistachios, oats, 1/4 cup of the flour, and 1/4 teaspoon each of the cardamom and salt in the bowl of food processor. Pulse until roughly ground, about 5 times. Add cold butter, and pulse until mixture resembles peas, about 10 times. Set aside.
Whisk together baking powder, baking soda, remaining 1 cup plus 2 tablespoons sugar, 2 1/2 cups flour, and 1/2 teaspoon each cardamom and salt in a large bowl. Whisk together buttermilk, melted butter, eggs, and vanilla in a separate large bowl until smooth. Fold buttermilk mixture into flour mixture until just combined. Gently fold cooled strawberries and rhubarb into batter.
Using a 3 oz. ice cream scoop, scoop about 1/3 cup batter into each muffin cup. Top each with about 1 tablespoon pistachio streusel. Bake in preheated oven until starting to rise and brown, about 8 to 10 minutes. Reduce temperature of oven to 350°F, and continue baking until a toothpick comes out clean, about 10 to 12 more minutes. Let cool in pan on wire racks.