01 - Preheat waffle iron according to manufacturer's instructions.
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 wet ingredients into dry ingredients. Stir gently until just combined; do not overmix as a few lumps are acceptable.
05 - Lightly grease waffle iron. Pour ½ to ¾ cup batter per waffle depending on iron size. Cook until golden and crisp, approximately 4–5 minutes per waffle.
06 - Combine blueberries, water, sugar, and lemon juice in a small saucepan. Bring to a simmer over medium heat, stirring occasionally. Cook until berries burst and syrup thickens, about 6–8 minutes. For thicker consistency, stir in cornstarch mixture and simmer 1–2 additional minutes.
07 - Serve waffles immediately, topped generously with warm blueberry syrup.