Coat a 6-quart slow cooker insert with nonstick spray.
Toss apples with granulated sugar, cornstarch, lemon juice, 1/2 tsp cinnamon, nutmeg, and salt. Spread evenly in cooker.
Rub oats, flour, brown sugar, remaining cinnamon, and butter into clumps; fold in pecans.
Scatter topping over apples without pressing down.
Stretch a clean cotton kitchen towel over cooker; cover with lid. Fold towel ends up over lid, away from heat.
Cook on high for 3 to 3 1/2 hours until apples are tender and juices bubble at the edge.
Remove lid and towel; rest 15 minutes before serving.