With the oven rack one level below the middle, heat the oven to 350°. Butter and flour a 9-by 5-inch loaf pan, knocking out the excess flour. Set the pan aside.
In a medium mixing bowl, whisk together the 2 cups flour, baking powder, baking soda, salt, and nutmeg. Set the mixture aside.
In the bowl of an electric mixer, beat the butter on medium-high for 2 minutes. Reduce the speed to medium, then gradually mix in the sugar. Add the eggs, one at a time, mixing well after each. Mix in the vanilla and lemon zest, scraping down the sides as needed.
Add half the dry mixture to the creamed ingredients, mixing on low to combine. Add the buttermilk, then stir in the remaining dry ingredients with a wooden spoon until evenly mixed. Stir the reserved 1 1/2 tablespoons flour with the apples to coat, then fold the apples into the batter.
Spread the batter evenly in the prepared pan, smoothing it with a spoon. Sprinkle with a teaspoon of sugar. Bake until a toothpick or wooden skewer inserted deep into the center comes out clean, 55 to 65 minutes. Let the loaf cool in the pan on a wire rack for 10 minutes, then loosen the loaf, turn it out, and let it finish cooling on the rack.
Variation: PEAR STREUSEL QUICK LOAF--Replace the apple with an equal amount of diced pears (about 4 medium pears). Combine 1/2 cup chopped pecans, 3 tablespoons sugar, 2 tablespoons flour, and 2 tablespoons cold unsalted butter, cut into pieces, in a large bowl. Rub well to mix. Spread the topping evenly over the batter and bake.
Variation: CRANBERRY-ORANGE QUICK LOAF--Replace the apples with 2 cups of coarsely chopped fresh cranberries. Toss with 2 1/2 tablespoons sugar; set aside for 30 minutes. Toss with 1 1/2 tablespoons flour, as above, and fold into the batter. Replace the lemon zest with 2 teaspoons orange zest. Bake.