Spray the bottom of the crockpot with cooking spray or brush it with melted butter so the apples release easily after several hours of cooking.
cooking spray
In a medium bowl, toss the apples with the granulated sugar, flour, cinnamon, lemon juice, and vanilla until every slice is coated. The flour thickens the juices as the apples cook, keeping the filling syrupy instead of watery. Transfer the apples to the crockpot.
5 medium (~ 900 g) Granny Smith apples, ½ cup (100 g) granulated sugar, 2 tbsps (16 g) all-purpose flour, 1 tsp (3 g) ground cinnamon, 1 tsp (5 ml) fresh lemon juice, 1 tsp (5 ml) vanilla extract
In another bowl, combine the oats, flour, brown sugar, cinnamon, and kosher salt. Work the cold butter into the mixture with a fork or your hands until crumbly, leaving a few larger butter pieces behind. They'll melt gradually as the crisp cooks and help create a better topping.
½ cup (50 g) old-fashioned oats, ½ cup (65 g) all-purpose flour, ¾ cup (150 g) packed light brown sugar , ½ cup (113 g) cold unsalted butter, 1 tsp ( 3 g) ground cinnamon, ¼ tsp (1 g) kosher salt
Sprinkle the topping evenly over the apples.
Lay a generous sheet of paper towels across the crockpot (2 full-size sheets or 3 connected half-sheets), making sure they cover the opening without sagging onto the topping. Place the lid over the towels and cook on HIGH for 1½ hours or LOW for 2½ to 3 hours. The paper towels catch the condensation before it drips back onto the topping, keeping it from turning soggy.
Remove the lid and carefully discard the paper towels. The apples should be bubbling around the edges. Continue cooking uncovered for 1 hour so the topping can finish crisping.
Turn off the heat or switch the crockpot to the warm setting and let the apple crisp rest for about 20 minutes before serving. The filling thickens as it cools, making it much easier to scoop. Serve on its own or with vanilla ice cream.
vanilla ice cream