In a 9-inch square baking dish or shallow 1 1/2- to 2-quart casserole, mix apples with granulated sugar, 2 tablespoons flour, and cinnamon.
In a bowl, combine rolled oats, brown sugar, butter, and 3/4 cup flour. Rub with your fingers until butter pieces are no longer distinguishable. Stir in chopped pecans. Sprinkle topping evenly over apples.
Bake dessert in the middle of a 350° oven until apples are tender when pierced and topping is browned, 45 minutes to 1 hour. If topping browns first, drape with foil.
Spoon the apple crisp, warm or cool, into bowls. Top portions with vanilla ice cream.