a rack in middle of oven and preheat to 325 deg F / 160 deg C. Grease a 9 x 5 inch loaf pan with
vegetable oil, and line with baking paper, leaving overhang on long
Pulse pistachios in a food processor til finely ground (it’s fine if some of the pieces are a bit larger). Set aside a heaping tablespoonful.
Whisk lemon zest, flour, sugar, baking powder, salt, and remaining ground nuts in a large bowl. Whisk olive oil, 2 tbsp lemon juice, remaining ⅓ cup vegetable oil, and ½ cup water in a small bowl to combine. Pour oil mixture into dry ingredients and fold together with a rubber spatula. Be very careful not to overmix: the batter is quite wet, which means it can get gummy if overworked. Pour into prepared tin and use a spatula to evenly distribute - it should come halfway up the sides.
Bake til golden brown all over, cake springs back when gently pressed,
and a tester inserted into the middle comes out clean, 50–60 minutes. Don't worry if it's dipped a little in the centre - that's totally fine. Transfer tin
to a wire rack and let cool completely in tin, at least 2 hours.
Using the overhang of baking paper, lift cake out of tin and place on rack. Remove baking paper.
Whisk icing sugar and 3 tbsp lemon juice in a medium bowl, drizzling in remaining 2 tbsp lemon juice as needed until you have a thick but pourable glaze. Pour over cake, letting it drip down the sides (if your cake has dipped, you can glaze the flat underside instead of the top if you like). Immediately sprinkle over reserved pistachios, then let sit til glaze is set, at least 30 minutes.
Because of the oils in the cake, it remains amazingly moist so can be made 3 days ahead.