In 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.

Heat the oven to 350°F. Lightly grease a 9-inch square baking pan.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt and nutmeg.

In 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 lemon juice and eggs, one at a time, scraping down the sides of the bowl after each addition.

Stir the flour mixture into the bowl, a third at a time, alternating with the oil, until both the flour mixture and oil are evenly incorporated into the batter. Gently fold 1 cup of the blueberries into the batter.

Spread 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.

Bake 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. Remove the cake from the oven and cool to room temperature. The cake will keep at room temperature for 2 to 3 days, covered in plastic wrap.