Preheat oven to 375 degrees F (190 degrees C). Lightly grease a 9x13-inch baking pan.
Place huckleberries and lemon juice in a bowl. Combine sugar, cornstarch, and 1 teaspoon cinnamon in a small dish; mix until well combined. Pour over the huckleberries; toss until completely coated. Pour into the prepared pan.
Combine brown sugar, flour, oats, nutmeg, and 1 1/2 teaspoons cinnamon in a bowl; toss together until completely mixed. Add butter; stir until mixture begins to form clumps. Spread over the huckleberries.
Bake in the preheated oven until golden brown, about 30 minutes. Allow to cool 15 minutes before serving.