Step 1

Place a rack in lower third of oven; preheat to 350°F. Toss plums, lemon zest, lemon juice, cornstarch, 1/2 cup brown sugar, 1/2 tsp. salt, and 1/4 tsp. cardamom in a large bowl; let sit until some juices accumulate, 5–10 minutes.

Step 2

Meanwhile, pulse flour and remaining 1/3 cup brown sugar, 1/2 tsp. salt, and 1/2 tsp. cardamom in a food processor to combine. Add butter and pulse until mixture is very sandy and starts to form larger clumps.

Step 3

Transfer plums and their juices to a 9"-diameter deep pie dish (all of those fruit juices will overflow a standard one) or an 8x8x2" baking dish. Scatter topping over fruit, squeezing small fistfuls in your hand to bring it together, then breaking into smaller pieces of varying sizes. Sprinkle pistachios evenly over topping.

Step 4

Bake crumble until juices are thickened and bubbling and top is golden brown, 40–45 minutes. Let cool slightly.