Peel, core, and slice the apples. Place them in a large bowl and toss with lemon juice, sugar, cinnamon, nutmeg, and a pinch of salt.
Spread the apple mixture evenly in the bottom of the crock pot.
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In a separate bowl, mix oats, flour, brown sugar, and cinnamon. Cut in the cold butter with a fork or pastry cutter until the mixture becomes crumbly. Stir in nuts if using.
Sprinkle the topping evenly over the apples.
Cover and cook on low for 3–4 hours or high for 2 hours, until apples are tender and bubbly.
For a slightly crispier topping, prop the lid open slightly during the last 30 minutes of cooking.
Serve warm with vanilla ice cream or whipped cream.