Prep the pan and oven. Heat the oven to 350°F (175°C). Line a 9x5-inch loaf pan with parchment, letting it overhang for easy lifting. Lightly grease the sides.

Whisk the dry ingredients. In a medium bowl, whisk flour, baking powder, baking soda, and salt. This keeps the crumb even and prevents clumps.

Combine the wet ingredients. In a large bowl, whisk sugar, eggs, and lemon zest until slightly thick and pale, about 30–45 seconds. Whisk in oil (or melted butter), yogurt, lemon juice, vanilla, and the sourdough starter until smooth.

Bring the batter together. Add the dry ingredients to the wet and fold gently with a spatula until just combined. If using poppy seeds, fold them in now. Do not overmix—a few small streaks of flour are fine.

Pan and smooth. Scrape the batter into the prepared pan and level the top. Tap the pan once on the counter to release air bubbles.

Bake. Bake 45–55 minutes, until the top is golden and a toothpick comes out with a few moist crumbs. If the top browns too quickly, tent with foil for the last 10 minutes.

Cool. Let the loaf cool in the pan for 10–15 minutes, then lift it out to a rack. Cool until just warm before glazing, or completely cool for a thicker glaze set.

Glaze. Whisk powdered sugar, lemon juice, and a pinch of salt until smooth and pourable. Drizzle over the loaf. Let it set 20–30 minutes before slicing.