Preheat oven to 375 degrees. Butter a 10" iron skillet. If you don't have an iron skillet, use a similar size oven-safe dish.
Crumble Topping
Combine oats, flour, brown sugar, salt, baking powder, and cinnamon in a medium bowl.
Cut in butter using a pastry cutter and/or your fingers until the mixture resembles coarse crumbs.
Fruit Filling
Cut up apples and sprinkle with lemon juice.
Cut up plums.
In a large bowl, combine brown sugar, flour, cinnamon, and salt.
Add fruit to the bowl and toss until all pieces are well coated.
Pour fruit mixture into prepared skillet.
Cover with crumble topping.
Bake for 35-45 minutes or until you can see the fruit mixture bubbling and the crumbs are browned. BE CAREFUL WHEN REMOVING SKILLET FROM OVEN. USE A POT HOLDER OR OVEN MITT TO GRAB THE HANDLE.
Serve warm topped with vanilla ice cream.
Notes
STORE/FREEZE/REHEATTo Store: Let the crumble cool completely and cover the crumble with plastic wrap and refrigerate it for up to 4 days.To Freeze: Cover the crumble with plastic wrap and then wrap it with aluminum foil. Freeze for up to 3 months. Thaw it in the fridge overnight.Note when freezing a crumble or crisp, the filling may get somewhat watery and the topping won't be as crunchy.To Reheat: The best way to reheat a fruit crumble is in the oven at 350°F until warmed through. You can also microwave it in 30-second intervals. Microwaved crumble topping won't be as crunchy.