How to Make It
Make the crumb topping In a medium bowl, whisk the sugar with the almonds, flour and salt. Using your fingers, rub in the butter until damp crumbs form. Refrigerate the topping until firm, about 15 minutes.
Meanwhile, make the cake Preheat the oven to 350°. Grease a 9-inch cake pan with nonstick spray and line with parchment paper. In a medium bowl, whisk 1 1/2 cups of the all-purpose flour with the almond flour, baking powder, salt and baking soda. In a large bowl, rub the lemon zest into the sugar until slightly damp and fragrant. Add the softened butter and, using a hand mixer, beat at medium speed until pale and fluffy, about 3 minutes. Add the egg, sour cream and vanilla and beat until just combined, about 1 minute. Add the dry ingredients and beat at low speed until just combined.
In a small bowl, toss the blueberries with the remaining 1 tablespoon of all-purpose flour. Using a rubber spatula, fold the blueberries into the cake batter. Spread the batter in the prepared pan, then sprinkle with the chilled crumb topping. Bake the cake for about 50 minutes, until golden brown and a toothpick inserted in the center comes out with a few moist crumbs attached. Transfer the cake to a wire rack and let cool for 30 minutes.
Run a thin knife around the edge of the cake and invert it onto a platter. Remove and discard the parchment paper. Invert the cake back onto the rack and let cool completely, about 1 hour. Cut the cake into thick wedges and serve with whipped cream.