Preheat the oven to 400 degrees F. Grease an 8-by-8-inch baking dish with some of the melted butter.
Peel and halve the apples; cut out the cores and place flat-side down. Using a sharp knife, make slits in each apple half 1/4 inch apart; do not cut through to the bottom. (If you make the slits too close together, the apples will fall apart). Place the apples flat-side down in the prepared baking dish.
Mix 2 tablespoons of the melted butter with 2 tablespoons of the brown sugar in a small bowl with a fork until a paste forms. Spread the mixture on the apples. Cover the baking dish with foil and bake until just tender when pierced with the tip of a knife, about 20 minutes.
Meanwhile, make the streusel. If the granola has big chunks in it, put it in a plastic bag and smash with the back of a measuring cup into smaller crumbles. Mix the granola with the flour, salt, remaining 1 tablespoon brown sugar and remaining 1 tablespoon butter in a bowl with a fork until the pieces are moistened and the mixture is clumpy.
Uncover the apples and spoon the streusel on the tops and in some of the slits, using a fork or the tip of a paring knife to gently separate the flaps. Return to the oven and bake, uncovered, until the apples are tender and the topping is golden, about 20 minutes more.
Microwave the caramel sauce in a small bowl until warm, about 45 seconds. Stir in the lemon juice. Transfer the apples to plates, drizzle with the caramel sauce and top with whipped cream.
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