Place flour, eggs, milk, water, melted butter, sugar, vanilla, and cinnamon in a blender. Blend for 15 seconds until smooth. Pour into a bowl, cover with a kitchen towel, and rest on the counter for 30 minutes.
While batter rests, make the apple filling: Melt 2 tablespoons butter in a medium skillet over medium heat. Add apple slices and cook for 2 minutes without stirring. Add brown sugar, maple syrup (if using), cinnamon, nutmeg, and salt. Stir to coat. Cook 4–5 minutes more, stirring occasionally, until apples are tender and liquid is syrupy. Stir in lemon juice. Transfer to a bowl.
Place skillet over medium-low heat. Add 1/2 teaspoon butter and swirl to coat. Stir the rested batter. Lift pan off heat, pour about 3 tablespoons (1/4 cup) batter into the center, then tilt and swirl to cover bottom evenly. Return to heat.
Cook for 45 seconds to 1 minute — edges will look dry and tiny bubbles appear. Center should still look slightly moist. Slide a thin spatula under one edge and flip. Cook second side for 15–20 seconds. Slide onto a plate and cover with a clean kitchen towel. Repeat with remaining batter, adding a tiny butter pat every other crepe.
To serve: Lay a crepe on a plate, place 2–3 tablespoons apple filling down the center, fold sides over or roll up. Dust with powdered sugar and a pinch of cinnamon. Serve immediately with whipped cream or yogurt and extra maple syrup. Repeat for remaining crepes.