• Center a rack in the oven and preheat the oven to 350 degrees F. Butter an 8 inch square pan and put it on a baking sheet.

for the walnut streusel:

• Put all the ingredients except the nuts in a food processor and pulse just until the mixture forms clumps and curds and holds together when pressed. Scrape the topping into a bowl, stir in the nuts, and press a piece of plastic against the surface. Refrigerate until needed. (Covered well, the crumb mix can be refrigerated for up to three days.)

for the coffee cake:

• Whisk together 2 cups of the flour, the baking powder, baking soda, salt, cinnamon, and nutmeg.
• Working in the bowl of a stand mixer fitted with the paddle attachment, or with a hand mixer, beat the sugar with the butter at medium speed until light, about three minutes. Add the eggs, one by one, beating for about 1 minute after each addition, then beat in the vanilla extract and lemon zest. Don't be concerned if the batter looks curdled- it will soon smooth out. Reduce the mixer speed to low and add the flour mixture and the buttermilk alternately, the flour in three parts and the buttermilk in 2 (begin and end with they dry ingredients.) *You will have a thick, creamy batter. Using your fingertips, toss the blueberries and 2 tsp of the flour together in a small bowl just to coat the berries. With a rubber spatula, gently stir in the berries.
• Scrape the batter into the buttered pan and smooth the top gently with the spatula. Pull the crumb mix from the fridge and, with your fingertips, break it into pieces. There's no need to try to get even pieces- these are crumbs, they're supposed to be every shape and size. Scatter the crumbs over the batter, pressing them down ever so slightly.
• Bake for 55-65 minutes, or until the crumbs are golden and a thin knife inserted in the center of the cake comes out clean. Transfer the cake to a rack and cool just until it is warm or until it reaches room temperature. Serve with warm berry sauce (see note below).