Peel, core and slice the pears into thin slices, about 1/4 inch thick. Place sliced pears into large bowl. Add the sugar, cornstarch, cinnamon, nutmeg, and salt; gently mix together.
Spread into pie crust; set aside. You will have a "mound" of pears in this pie. This is normal, the pears will bake down and be perfect.
In a medium bowl, combine the cold butter (cut into small cubes), flour, and brown sugar with two butter knives, pastry knife or clean hands (a small food processor works well, too) until you have coarse crumbs. Sprinkle crumb mixture over the pears. I lightly pack the crumb mixture down so it will stick to the pears and not too much falls off.
Bake pie for 15 -20 minutes. Then check to see if the topping is getting too brown. If so, cover with foil and bake another 20 minutes or until pears are tender and cooked through. If the pie isn't brown enough, no need to cover it at this point, but check it after about another 5 minutes.
Allow to cool completely on wire rack before slicing. I like to chill the pie once it's cooled before slicing so that the filling sets a bit but it's not required.