Preheat oven to 350℉.
Toss plums, lemon zest and juice, cornstarch, ½ cup brown sugar, ½ teaspoon salt, ½ cardamom, and ½ teaspoon cinnamon in a large bowl.
Using a food processor, pulse together the remaining brown sugar, sea salt, spices, almond flour, and almonds until fine. Pulse in the butter until mixture starts to form larger clumps.
Transfer the plum mixture to a deep 13 x 9-inch baking dish or deep 10-inch pie plate.
Scatter the topping evenly over the plums.
Bake for about 35 minutes or until filling is bubbling and crumble topping is golden brown and crispy.
Remove from oven and cool slightly before serving.
Delicious with some cardamom ice cream on the side!