Lightly grease the bottom and sides of a 5–6 quart slow cooker with butter or nonstick spray to help prevent sticking and make cleanup easier.
nonstick cooking spray or butter
Peel, core, and slice the apples about 1/4-inch thick, keeping the slices as even as possible so they cook at the same rate. If working slowly, toss the slices with a little lemon juice as you go to prevent browning.
6-7 medium apples, 1 tablespoon lemon juice
In a large bowl, combine the sliced apples, 1/2 cup brown sugar, 1 teaspoon cinnamon, nutmeg (if using), lemon juice, vanilla, and cornstarch. Toss until every slice is well coated and no dry patches of cornstarch remain.
6-7 medium apples, 1/2 cup light brown sugar, 1 teaspoon ground cinnamon, 1/4 teaspoon ground nutmeg, 1 tablespoon lemon juice, 1 teaspoon vanilla extract, 2 tablespoons cornstarch
Spread the apple mixture evenly in the prepared slow cooker, pressing down lightly so there are no large air pockets. This helps the apples cook down into a soft, saucy layer.
6-7 medium apples
In another large bowl, stir together the oats, flour, 1/2 cup brown sugar, granulated sugar, 1/2 teaspoon cinnamon, and salt. Add the cold butter cubes and work them into the dry ingredients with your fingers or a pastry cutter until the mixture resembles coarse crumbs with some pea-sized bits of butter. Stir in the chopped nuts if using.
1 cup old-fashioned rolled oats, 3/4 cup all-purpose flour, 1/2 cup light brown sugar, 1/4 cup granulated sugar, 1/2 teaspoon ground cinnamon, 1/4 teaspoon fine sea salt, 1/2 cup unsalted butter, 1/2 cup pecans or walnuts
Sprinkle the oat mixture evenly over the apples in the slow cooker. Do not pack it down firmly; leaving it a bit loose allows steam to circulate and helps the topping crisp up.
1 cup old-fashioned rolled oats, 3/4 cup all-purpose flour, 1/2 cup light brown sugar, 1/4 cup granulated sugar, 1/2 teaspoon ground cinnamon, 1/4 teaspoon fine sea salt, 1/2 cup unsalted butter, 1/2 cup pecans or walnuts
Cover and cook on LOW for 3–4 hours, or on HIGH for about 2 hours, until the apples are very tender when pierced with a fork and the topping looks set and lightly golden around the edges. Slow cookers vary, so start checking a bit early the first time.
For the last 20–30 minutes of cooking on LOW, prop the slow cooker lid slightly open with a wooden spoon if possible. This allows steam to escape and helps the topping firm up instead of becoming too soft.
Turn off the slow cooker and let the apple crisp rest for 10–15 minutes to thicken slightly. Spoon into bowls and serve warm with vanilla ice cream, whipped cream, or a dollop of vanilla Greek yogurt, if desired.
vanilla ice cream, whipped cream, or vanilla Greek yogurt