Peel the apples and carefully scoop out the cores. You can remove a little extra from the center—the filling will set nicely when chilled. Set aside a peel from one apple for later.
In a small pot, heat the heavy cream and butter. Once melted and hot (but not boiling), stir in the ground walnuts and hazelnuts. Mix well and let it cool to room temperature.
In a large pot, combine sugar, 2.2 cups (500 ml) of water, and a few lemon slices. Bring to a boil, then gently add the apples in the pot.
Let the apples simmer on low heat for 3–5 minutes on one side, then turn and cook another 3–5 minutes. Check with a toothpick—it should slide in easily but the apples shouldn’t fall apart.
Carefully take the apples out and let them cool to room temperature. This makes them easier to handle and fill.
Continue simmering the syrup for 15–20 minutes until thickened. Add the saved apple peel while it cooks to help it thicken naturally.
Roll the cooled apples in ground walnuts and set them on a tray or plate, ready for filling.
Spoon the walnut and hazelnut mixture into each apple, pressing it in gently so the center is evenly filled. Once the syrup has cooled a bit, pour it over the apples. Chill in the fridge until ready to serve.
Right before serving, top with whipped cream using a piping bag. Finish with a sour cherry, walnut, or your favorite topping.
Enjoy!