Place unopened can of sweetened condensed milk in a large pot; fill pot with water to cover. Bring water to a simmer, and simmer can for 3 hours, adding hot water as needed to keep can covered. Remove can from water with tongs; allow to cool, about 20 minutes.
Place apples in a large bowl. Whisk 1 tablespoon flour, cinnamon, and nutmeg together in a small bowl; stir mixture into apples. Pour apple mixture into a 9x13-inch baking pan. Stir vanilla extract into water in a bowl; sprinkle over apple mixture.
Open cooled condensed milk carefully. Stir caramelized milk until smooth; drizzle evenly over apple mixture.
Stir oats, 1 cup flour, brown sugar, baking soda, and salt together until combined; stir in butter until crumbly. Spread crumble mixture over apples and caramel.
Bake in the preheated oven until apples are tender and crumble is lightly browned, about 25 minutes.
You can also cook the sweetened condensed milk in a slow cooker. Place the can in slow cooker with water to cover; cook on High for 1 hour, then on Low for 4 hours. You can cook more than one can at a time; store unopened cans in the pantry.
Aluminum foil helps keep food moist, ensures it cooks evenly, keeps leftovers fresh, and makes clean-up easy.