Position a rack in the center of the oven, set a heavy-duty rimmed baking sheet on the rack, and heat the oven to 350°F.
Make the streusel
In a medium bowl, combine the flour, oats, sugar, and salt. Using your fingers, blend the butter into the flour mixture. The mixture will be moist. Set aside.
Make the filling
In a large bowl, toss the pears with the lemon juice. In a small bowl, whisk the sugar, flour, salt, cinnamon, and nutmeg. Add the sugar mixture to the pears and toss well to combine. Stir in the cherries.
Mound the filling into the piecrust. Sprinkle the streusel topping over the pear mixture, pressing the streusel between your fingers into small lumps as you sprinkle.
Put the pie on the heated baking sheet and bake until the pastry is golden-brown and the filling is bubbly and thickened at the edges, 55 to 65 minutes. Rotate the pie halfway through baking, and if the pastry or streusel browns before the filling has thickened, loosely cover the top or edges of the pie as needed with a pie shield or a sheet of aluminum foil.
Transfer to a rack and cool completely before serving. The pie can be stored at room temperature for up to 2 days.
nutrition information (per serving):
Calories (kcal): 630; Fat (g): 24; Fat Calories (kcal): 210; Saturated Fat (g): 15; Protein (g): 7; Monounsaturated Fat (g): 6; Carbohydrates (g): 101; Polyunsaturated Fat (g): 1; Sodium (mg): 260; Cholesterol (mg): 60; Fiber (g): 7.