Heat the oven to 350°F (175°C). Butter the dish and set it on a rimmed sheet. Choose firm ripe pears without soft bruised patches.
Peel, core, and slice pears evenly. Toss with fruit sugar, lemon juice, cornstarch, and fresh ginger so the flavor spreads through the filling.
Arrange pears in an even layer in the dish. Scrape the bowl to include the measured thickener and any fruit juices.
Mix topping flour, oats, brown sugar, ground ginger, and salt. Cut in cold butter until rough crumbs and small clusters form.
Scatter the topping loosely over the pears. Leave a few gaps near the center where you can watch for bubbling juices.
Bake 40–50 minutes until topping is golden and a central pear slice is tender. Fruit juices should bubble rather than remain thin and still.
Cover the topping loosely with foil if it darkens too fast. Keep baking until fruit tenderness and bubbling indicate the filling is cooked.
Let stand 20 minutes before serving warm. Use a broad spoon to include some topping with the soft pears in each bowl.
