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Adjust rack to middle position and preheat oven to 400 degrees. Grease an 8-by-8-inch nonstick baking dish (9-by-13-inch baking dish for 12 servings) with nonstick cooking spray. Reserve 1 TBSP flour (2 TBSP for 12) in a large bowl. Wash and dry produce.
Place 4 TBSP butter (8 TBSP for 12) in a medium microwave-safe bowl; cover with plastic wrap. Microwave until melted, 60-90 seconds. (Get a head start peeling apples while butter melts!)
To bowl with melted butter, add oats, pecans, half the cinnamon, remaining flour, 4 TBSP white sugar (8 TBSP for 12), and a pinch of salt. Mix with a fork to combine. Place streusel in freezer to firm up while preparing filling.