Step 1In a medium bowl, whisk together the flour, sugar, salt and lemon zest. Add the butter, using a fork or your fingers to cut in the butter until it is reduced to the size of peas. Loosely cover the bowl, and place it in the freezer while you mix the cake batter.
Step 1Heat the oven to 350 degrees. Lightly grease a 9-inch square baking pan.
Step 2In a medium bowl, whisk together the flour, baking powder, baking soda, salt and nutmeg.
Step 3In the bowl of a stand mixer, or in a large bowl with a hand mixer, cream together the butter, three-fourths cup sugar and lemon zest until light and fluffy, 3 to 5 minutes. Add the eggs, one at a time, scraping down the sides of the bowl after each addition.
Step 4Stir the flour mixture into the bowl, a third at a time, alternating with the buttermilk, until both the flour mixture and buttermilk are evenly incorporated into the batter. Gently fold 1 cup of the blueberries into the batter.
Step 5Spread the batter into the prepared pan and distribute the remaining blueberries evenly over the top of the batter. Remove the crumb topping from the freezer and sprinkle it over the berries.
Step 6Bake the cake until it is lightly golden and firm on top, and a toothpick inserted in the center comes out clean, 45 to 50 minutes. Rotate the pan halfway through for even baking.
Step 7While the cake is baking, make a lemon syrup: In a small saucepan, combine the remaining one-third cup sugar with the lemon juice and whisk until blended. Heat the pan over medium-low heat and cook, stirring occasionally, until the liquid thickens to a syrupy consistency, 6 to 8 minutes. (The glaze will bubble while cooking and may need to be removed from the heat to check that it is the proper consistency.) Remove from heat and set aside in a warm place.
Step 8Remove the cake from the oven and drizzle the warm glaze over. Cool to room temperature. The cake will keep at room temperature for 2 to 3 days, covered in plastic wrap.