01 - Preheat your waffle iron according to manufacturer's instructions until ready for baking.
02 - In a large bowl, whisk together flour, poppy seeds, sugar, baking powder, baking soda, and salt until well blended.
03 - In a separate bowl, whisk together milk, eggs, melted butter, lemon zest, lemon juice, and vanilla extract until smooth.
04 - Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix; a few small lumps are acceptable.
05 - Lightly grease the waffle iron. Pour batter onto the hot waffle iron, using about ½ to ¾ cup per waffle depending on size. Cook until golden and crisp, approximately 4–5 minutes each.
06 - In a small saucepan, combine blueberries, water, sugar, and lemon juice. Bring to a simmer over medium heat, stirring occasionally, until berries burst and syrup thickens, about 6–8 minutes. For thicker syrup, stir in cornstarch slurry and simmer 1–2 minutes more.
07 - Serve waffles warm, topped generously with homemade blueberry syrup.