For the topping: Combine the ingredients in a medium bowl and work together with a fork or with your fingertips until the ingredients are moistened.
For the filling: Combine the pears slices, lemon juice, and ginger in a large bowl. Add the sugar and flour and toss to blend.
Spray an 8 by 8-inch baking dish with cooking spray. Transfer the pear mixture to the dish. Sprinkle the topping over the pears. Bake crumble until the pears are tender and the topping is golden brown, about 40 minutes. Let cool for 10 minutes before serving. Serve warm or at room temperature, with a scoop of ice cream, if desired.