01 - Preheat your 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 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 about ½ to ¾ cup batter per waffle onto the hot iron and cook until golden and crisp, approximately 4-5 minutes per waffle.
06 - While waffles cook, 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 slurry and simmer 1-2 additional minutes.
07 - Serve waffles immediately, topped generously with warm blueberry syrup.