This cake feels to me 100 percent Californian, the land of abundant avocados. Use a good-quality avocado, and make sure it's not too ripe--just ripe enough to be sweet, and bright green where the flesh meets the skin.
Make the cake: Position a rack in the center of your oven and preheat the oven to 350 degrees F. Butter a 9-by-5-inch loaf pan and line the bottom and long sides with parchment paper, leaving a 1/2-inch overhang.
Sift the flour, baking powder and baking soda into a mixing bowl. Add the ground pistachios and salt and whisk to combine. Set aside.
In the bowl of a stand mixer fitted with the whisk attachment, beat the sugar and eggs together on medium speed until the eggs loosen, then slowly stream in the oil until it emulsifies. Continue to beat until the mixture is very light in color and creamy, about 2 minutes. Add the avocado, sour cream and zest and beat just until combined. Scrape down the sides and bottom of the bowl. Incorporate the flour mixture in three additions, mixing on low speed after each addition just until no bits of dry flour remain.
Transfer the batter to the prepared loaf pan and lightly rap the pan on the counter to dislodge any air pockets in the batter. Bake for about 1 hour, until a cake tester inserted into the center of the cake comes out with just a few moist crumbs attached. Allow the cake to cool completely in its pan.
While the cake cools, make the glaze: Sift the confectioners' sugar into a small mixing bowl. Add the lemon zest, ground pistachios and 2 tablespoons of the lemon juice and mix until incorporated. Add the remaining 1 tablespoon lemon juice and mix until the thickness of the glaze is similar to runny honey. If you want a thinner consistency, add additional lemon juice or water 1 teaspoon at a time and whisk until combined.
Set a wire rack over a parchment-lined baking sheet and turn the cake out onto the rack. Pour the glaze over the top. Decorate the cake with candied lemon zest, edible flowers and toasted pistachios, if desired. Let the glaze set on the cake for 20 minutes before cutting. This cake keeps well in a sealed container at room temperature for 1 week.