Heat the oven to 375°F. Peel and core the pears, then cut them into 1/3-inch slices so they soften at a similar rate.
Toss pears with lemon juice, granulated sugar, cornstarch, 1/2 teaspoon cinnamon, ginger and half the salt. Spread in an 8-inch square dish set on a sheet pan.
Mix oats, flour, brown sugar, remaining cinnamon and salt, breaking up any hard sugar lumps.
Work in cold butter and vanilla with a pastry cutter or fingertips until moist clumps form and some pea-size butter pieces remain.
Scatter the topping loosely over the pears and bake 38 to 48 minutes, until deeply golden and the fruit juice bubbles visibly at several edges.
Rest at least 15 minutes so the thickened juices settle and the topping firms.
