Make the filling
Preheat the oven to 350°F (180ºC). Add apples to a large bowl. Pour the granulated sugar, 2 tablespoons of flour and cinnamon over the apples and toss until all the slices are evenly coated.
Arrange the apples in the pan
Spread the apple mixture evenly in a 9 x 13” glass baking dish or another similar size pan. Pour ¼ cup water evenly over the top gently moving the apples around in the pan to coat them. This helps to keep the mixture moist.
Make the oat topping
In a medium mixing bowl combine the oats, remaining cup of flour, brown sugar, baking powder, baking soda, and melted butter together.
Bake the apple crisp
Crumble the oat topping evenly over the apple mixture. Bake the crisp until the top of the apple crisp is golden brown, about 50 to 60 minutes.
Let cool for about 10 minutes. Slice the warm apple crisp into squares and use a spatula to scoop into bowls topped with vanilla ice cream. Serve warm.