For topping, stir together butter and ANGOSTURA® aromatic bitters . In a medium bowl stir flour, oats and brown sugar. Drizzle butter over flour mixture as evenly as possible and stir well. Refrigerate 30-60 minutes. When ready to assemble crisp, remove topping from refrigerator and crumble with your fingers.
Preheat oven to 350°F. Core apples; quarter and slice about 1/4” thick. You should have 5 cups apples, about 1-1/3 lb. total. In a 9-inch deep dish pie plate or square baking dish, toss together apples and a generous sprinkle of cinnamon. Sprinkle with 1 tablespoon. brown sugar and toss again. Scatter topping over apples. Bake 35 minutes. Let cool 1 hour, then spoon onto plates or bowls. Top with a scoop of vanilla ice cream, if desired.