Coat a 6-quart slow cooker insert with nonstick spray.
Toss the apples with the granulated sugar, cornstarch, lemon juice, 1/2 tsp cinnamon, nutmeg and salt and spread them evenly in the cooker.
Rub the oats, flour, brown sugar, remaining cinnamon and butter together until clumps form, then fold in the pecans.
Scatter the topping over the apples without pressing it down.
Stretch a clean cotton kitchen towel over the cooker, set the lid on top and fold the towel ends up over the lid away from the heat.
Cook on high until the apples are tender and juices bubble at the edge, 3 to 3 1/2 hours.
Remove the lid and towel and let the crisp stand for 15 minutes before spooning it out.