Preheat the oven to 375°F (190°C). Line a large baking sheet with parchment paper to prevent sticking and ensure even baking.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, cinnamon, nutmeg, and salt until the mixture looks uniform and the spices are evenly distributed.
In a medium bowl, whisk the milk, eggs, vanilla extract, and melted butter until smooth. Make sure the butter is not hot so it blends evenly without curdling.
Pour the wet ingredients into the dry ingredients. Stir gently with a spatula just until the flour disappears. The batter should be thick and slightly lumpy. Overmixing will make the fritters dense instead of soft.
In a small bowl, toss the diced apples with brown sugar and cinnamon until fully coated. This step ensures even sweetness and soft apple pieces after baking.
Fold the apple mixture into the batter using slow, gentle turns. Stop mixing as soon as the apples are evenly distributed to maintain a tender texture.
Scoop the batter into 10 to 12 mounds on the prepared baking sheet, leaving space between each fritter so heat can circulate properly.
Bake for 22 to 25 minutes, or until the tops are lightly golden and the centers feel set when gently pressed. If needed, bake an additional 1 to 2 minutes.
Remove the fritters from the oven and let them rest on the baking sheet for 5 minutes. This allows the interior to finish setting while staying soft.
If using the glaze, whisk powdered sugar, milk, and vanilla until smooth. Drizzle lightly over warm fritters for brunch service.