Stir together with a fork until all of the cornstarch is dissolved. Add the sugar and the cinnamon to the cornstarch mixture and whisk it together until it's smooth.
Add the sliced pears to a large bowl. Pour the cornstarch mixture over the pears and gently stir until the pears are coated. – Pour the pear filling into the cast-iron skillet.
In a large bowl combine the brown sugar, flour, nutmeg, and oats. Stir together. Add the cold diced butter. Using a pastry cutter, cut the butter until crumbly.