Preheat your oven to 425°F (220°C). Line a jumbo 6-cup muffin tin with paper liners or spray generously with non-stick cooking spray.
Peel, core, and dice the apples into approximately ¼-inch pieces. Toss them with 1 tablespoon of lemon juice to prevent browning and enhance flavor.
In a medium bowl, whisk together 1 cup flour, ⅓ cup granulated sugar, ⅓ cup brown sugar, 1 teaspoon cinnamon, and ⅛ teaspoon salt for the crumb topping. Pour in the melted butter and stir with a fork until the mixture forms crumbs of varying sizes. Set aside.
In a large bowl, whisk together the flour, baking powder, cinnamon, nutmeg, and salt.
In another bowl, whisk the melted butter and granulated sugar until well combined. Add the eggs one at a time, beating well after each addition. Stir in the buttermilk and vanilla extract.
Pour the wet ingredients into the dry ingredients and gently fold together with a spatula until just combined. Some small streaks of flour are fine.
Fold in the diced apples until evenly distributed.
Divide the batter evenly among the 6 jumbo muffin cups. They should be filled all the way to the top.
Generously sprinkle the crumb topping over each muffin cup, using your fingers to gently press it into the batter slightly. Use all of the topping – about 2-3 tablespoons per muffin.
Bake at 425°F (220°C) for 5 minutes. Then, without opening the oven door, reduce the temperature to 350°F (175°C) and continue baking for 20-25 minutes more, until a toothpick inserted in the center comes out clean.
Let the muffins cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
If using the glaze, whisk together powdered sugar, milk or apple cider, and vanilla extract until smooth. Drizzle over completely cooled muffins.