In a large bowl, whisk together starter, milk, yogurt, egg, melted butter, and sugar until smooth. Stir in lemon zest and poppy seeds. Add flour and salt; mix with a spatula or your hands until no dry flour remains. Cover and let rest for 30 minutes.

With wet hands, perform a set of stretch and folds: grab one side of dough, stretch up and fold over center, rotate and repeat all four sides. Cover and rest 30 minutes. Repeat folding process 3 more times at 30-minute intervals (4 total sets). After final fold, let dough ferment until nearly doubled, 2 to 3 hours more.

Turn dough onto floured surface, pat into a rectangle, roll tightly into a log, pinch seam. Place seam-side down in greased 9x5-inch loaf pan. Cover and rest 15 minutes.

Cover and proof until dough crests about 1 inch above pan rim, 1.5 to 2 hours. Dough should look puffy and a gentle poke springs back slowly.

About 30 minutes before baking, preheat oven to 375°F (190°C). Slash loaf down center, bake 45-50 minutes until deep golden and internal temp 190-195°F (88-91°C). Cool in pan 10 minutes, then turn out onto rack to cool completely.

In a small bowl, whisk together 1/2 cup powdered sugar and 1 tbsp lemon juice until smooth, adding more juice if needed for a pourable consistency. Drizzle over cooled loaf and let set 10 minutes. Slice with serrated knife and serve.