In the bowl of a stand mixer fitted with a hook attachment, stir to combine yeast, water, milk, and sugar.
Add bread flour and salt, and knead on low to medium speed for 5–6 minutes, until smooth.
Add butter a pat at a time, kneading to combine between each addition.
Once all of the butter has been added, continue to knead for 5–6 minutes, until the dough is smooth and supple.
Remove the bowl from the mixer and cover with plastic wrap or a tea towel. Set aside to rise for 1 ½ to 2 hours, or until doubled in size.
In the meantime, prepare the filling: Heat butter in a small pot over medium heat until melted.
Add diced apples, maple syrup, and brown sugar and cook, stirring, for 4–5 minutes, or until the apples are tender but aren't falling apart.
Remove from the heat. Add lemon juice, flour, and cinnamon, and stir until thickened.
Transfer the mixture to a bowl and set aside to cool to room temperature.
Spray a standard loaf pan with nonstick cooking spray; set aside.
Turn the dough out onto a flour-dusted surface and roll to a 9x18-inch rectangle.
Spread the apple filling evenly over the surface, leaving a clean, ½-inch border around the edges. Sprinkle evenly with raisins or walnuts, if desired.
Starting on one of the short sides, roll the dough tightly to create a 9-inch log.
Pinch the seams and press to seal the ends.
Transfer to the loaf pan, cover, and set aside to rise for 45–60 minutes, or until puffed and marshmallowy.
Preheat the oven to 375 F.
Whisk to combine the egg with a splash of water.
Brush the egg wash over the loaf.
Bake the loaf for 45–50 minutes, or until the top is browned and the internal temperature registers at least 190 F. Tent with foil, if needed, to prevent excess browning.
Cool the loaf in the pan for 10 minutes, then turn out onto a wire rack to cool completely.