In a medium bowl, add 2 cups huckleberries, arrowroot and salt. With a rubber spatula, gently stir until coated being careful not to crush the berries. Set aside.
In a small bowl, add butter, vinegar, zest, warm honey and syrup. Drizzle over huckleberry mixture and gently fold in with a spatula to combine. Let sit for 15 minutes.
In a food processor, pulse flour, nuts, brown sugar, salt, cinnamon and ¾ cup oats in a food processor to blend. Pulse in butter until no dry spots remain. Transfer to a bowl and work in remaining ¼ cup oats with your fingers.
Grease pan.. A standard 8x8 pan or small cast iron skillet will work. I used a vintage pan, 10” x 6.5” x 2.5”.
Layer bottom of pan with plums.
Stir huckleberry mixture one last time. Pour over plums evenly.
Top with crumble topping.
Bake in oven at 375 for 50-65 minutes or until berries are bubbling and topping is lightly browned. Remove and cool. Store in fridge and enjoy within 1-3 days. It’s totally delicious with ice cream!