TOPPING: In a large bowl, toss together brown sugar, oats, flour, and salt. Add butter and mix with your hands until the mixture comes together in moist clumps. Cover; chill while preparing filling.
FILLING: Preheat oven to 375 degrees. Generously butter 13x9x2-inch glass baking dish OR 6-8 mini skillets or ramekins.
In a large heavy pot, combine cranberries, apples, sugar, and apple juice. Bring to boil over medium heat, stirring often. Boil until cranberries are tender and juices start to thicken, about 5 minutes. Transfer filling to prepared dish(es). Sprinkle with topping.
Bake crisp(s) until filling is thick and bubbling and the topping is crisp and deep golden brown, about 35-40 minutes. Let cool 10 minutes. Serve warm topped with vanilla ice cream.