Prep oven and loaf pan: Preheat your oven to 350°F (175°C). Grease a standard 9x5-inch metal loaf pan and line with parchment paper, leaving an overhang on the sides for effortless removal.
Toss apples: In a small bowl, toss the finely diced Honeycrisp apples with 2 tablespoons of brown sugar and 1 teaspoon of ground cinnamon until evenly coated. Set aside.
Make the streusel topping: In another small bowl, mix 1/3 cup brown sugar, 1/4 cup flour, and 1 teaspoon cinnamon. Cut in 3 tablespoons of cold cubed butter using a pastry cutter or fork until coarse crumbly clumps form. Chill in refrigerator while making the batter.
Cream butter and sugar: In a large mixing bowl or stand mixer, beat the softened butter and granulated sugar together on medium-high speed for 2 to 3 minutes until pale, light, and fluffy. Beat in the eggs one at a time, followed by vanilla extract.
Combine wet and dry: In a separate bowl, whisk together the flour, baking powder, sea salt, and nutmeg. Add half the dry ingredients to the butter mixture, followed by the whole milk, and finish with the remaining dry ingredients. Mix on low speed just until combined (do not overmix).
Layer batter and apples: Spread half of the thick batter evenly into the bottom of the prepared loaf pan. Scatter half of the cinnamon spiced apples over the batter. Spread the remaining batter on top, then crown with the remaining spiced apples. Scatter the chilled crunchy streusel evenly across the entire surface, pressing very gently so it adheres.
Bake to golden perfection: Bake at 350°F for 50 to 55 minutes, or until a wooden skewer or toothpick inserted into the center of the loaf comes out clean with a few moist crumbs (no wet batter). If the streusel browns too quickly near minute 40, loosely tent the top with aluminum foil.
Cool, glaze and serve: Let the bread cool in the pan on a wire rack for 15 minutes, then use the parchment handles to lift the loaf onto the rack to cool completely. In a small bowl, whisk powdered sugar, milk, and vanilla until smooth. Drizzle generously over the crunchy streusel, let set for 5 minutes, slice thickly, and enjoy!