Heat the oven: Heat the oven to 375°F and butter a 9-inch baking dish.
Prep the pears: Peeling, coring, and slicing the pears takes about 10 minutes. Toss the slices with the lemon juice, sugar, cornstarch, cinnamon, ginger, and nutmeg, then spread them in the dish.
Make the topping: Rub the oats, flour, brown sugar, butter, and salt together with your fingertips until crumbly, about 5 minutes.
Assemble: Scatter the topping evenly over the pears, pressing it lightly so it stays put.
Bake: Bake 40 minutes, until the filling bubbles at the edges and the topping is golden and crisp.
Serve: Rest 10 minutes so the filling sets, then spoon into 6 bowls. Serve warm, with cream or ice cream if you like.