Peel, core, and slice the apples into 1/3–1/2-inch slices and set aside.

In a 12-inch skillet over medium heat combine the apples, 1/2 cup granulated sugar, 2 tablespoons lemon juice, 1 teaspoon vanilla extract, 1/2 teaspoon cinnamon, and a pinch of salt; cook 5–7 minutes until the sugar dissolves and apples begin to release juice.

Stir together 1 tablespoon cornstarch and 1 tablespoon cold water, add to the skillet, cook 30–60 seconds until the mixture slightly thickens, then remove from heat.

Transfer the macerated apples and juices to the slow cooker and spread into an even layer.

In a bowl mix 1 cup rolled oats, 1/3 cup brown sugar, 3 tablespoons melted butter, and 1/2 teaspoon cinnamon; sprinkle this mixture evenly over the apples and gently press it into the fruit.

Cover and cook on LOW for 2 hours.

While the crisp cooks, combine 1 1/4 cups rolled oats, 1/2 cup flour, 1/3 cup brown sugar, 1/2 teaspoon salt, 1 teaspoon cinnamon, and 6 tablespoons cold cubed butter; cut in until coarse crumbs form.

After 2 hours, remove the lid, sprinkle the final topping evenly over the fruit, increase to HIGH, and continue cooking uncovered or with the lid slightly ajar for 45–60 minutes until the topping is golden and the filling bubbles at the edges.

Let rest 10–15 minutes before serving so the sauce thickens; serve warm with vanilla ice cream or whipped cream if desired.