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.
