Place rack in lower third of oven; preheat to 350 degrees F (176 degrees C). Spray a 10-inch deep dish pie plate or 8x8x2-inch baking dish with non-stick cooking spray; set aside.
In a medium bowl, toss the Honeycrisp Apple (2 pound) and Plum (2 pound) with Lemon Juice (2 tablespoon).
In a small bowl, whisk together the Old Fashioned Rolled Oats (1/4 cup), Brown Sugar (1/2 cup), Corn Starch (1/4 cup), Kosher Salt (1 teaspoon), Ground Cinnamon (1 teaspoon) and Ground Cardamom (1/2 teaspoon). Sprinkle oat mixture on fruit slices; gently stir and let sit until juices start to accumulate, about 5-10 minutes.
Make the topping: In a medium bowl, mix together the Old Fashioned Rolled Oats (3/4 cup), All-Purpose Flour (1/2 cup), Brown Sugar (1/2 cup) and Ground Cardamom (1/2 teaspoon).
Add Unsalted Butter (4 tablespoon) and using a fork, mix together until it is well-blended and crumbly.
Transfer apples and plums to baking dish and spread evenly. Scatter topping over fruit. Sprinkle Pistachios (1/3 cup) evenly over topping.
Bake until juices are thickened and bubbling, apples are tender and the top is golden brown, about 40-50 minutes.
Let cool slightly and serve warm. You can always serve with ice cream, but this stuff is pretty spectacular all on its own!