Preheat the oven to 350°F (180°C) and butter a 2-quart baking dish. Set aside.

Peel, core, and slice the pears 1/4-inch (6-mm) thick and add to a large bowl as you go.

Stir in the brown sugar, tapioca starch, lemon juice, cinnamon, and vanilla extract, then pour into the prepared baking dish.

In the now empty mixing bowl, stir together the flour, brown sugar, walnuts, and cinnamon, then mix in the melted butter until combined.

Crumble the mixture over the pears, then place on a baking sheet and bake for about 45 minutes, until the topping is browned and the pears are bubbling. (If the topping starts to overbrown before the filling is done, cover loosely with aluminum foil for the remaining baking time.)

Let cool for 15 minutes, then serve while still warm with a scoop of vanilla ice cream.

Store leftovers, loosely covered, at room temperature for up to 2 days.