Preheat oven & prep dish: Preheat your oven to 350°F (175°C). Lightly butter a 9-inch round ceramic pie dish or a 2-quart baking casserole.

Toss pear filling: In a large mixing bowl, toss the sliced pears with fresh lemon juice, brown sugar, arrowroot powder (or flour), ground cinnamon, nutmeg, cardamom, and pure vanilla extract until every pear slice is evenly coated in spiced syrup. Transfer evenly into the prepared baking dish.

Combine dry crumble: In a separate medium bowl, whisk together the rolled oats, unbleached flour, dark brown sugar, cinnamon, and fine sea salt until no sugar clumps remain.

Cut in butter: Add the cold cubed butter. Using your fingertips or a pastry blender, rub the butter into the oat mixture until large, pea-sized buttery crumb clusters form. Do not overwork into a paste.

Scatter topping: Scatter the oat crumble topping generously and evenly across the spiced pear layer, ensuring full coverage all the way to the rim.

Bake undisturbed: Bake in the center of the preheated oven for 35 to 40 minutes, until the fruit juices are vigorously bubbling around the edges and the oat crumble is deep golden brown and fragrant.

Rest and serve: Remove from the oven and let rest on a wire rack for 10 minutes so the caramel pear juices settle and thicken slightly. Spoon warm into shallow bowls and crown immediately with a cold scoop of vanilla bean ice cream!