Preheat the oven to 350°F (175°C) and set a rack in the center.
Line a 9 x 5 inch loaf pan with parchment paper, leaving a little overhang, and lightly grease any exposed pan surface.
In a mixing bowl, whisk together the flour, sugar, poppy seeds, baking powder, baking soda, salt, and lemon zest until evenly combined.
In a second mixing bowl, whisk together the eggs, Greek yogurt, milk, oil, lemon juice, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and gently fold with a rubber spatula just until no dry streaks remain.
Scrape the batter into the prepared loaf pan and smooth the top lightly.
If using the optional topping, sprinkle the coarse sugar and poppy seeds evenly over the batter.
Bake until the top is set and a toothpick inserted in the center comes out clean or with a few moist crumbs, about 45 to 55 minutes.
Cool the loaf in the pan for 15 minutes, then lift it out using the parchment overhang and transfer it to a wire cooling rack to cool completely.
Slice once fully cooled and serve.