01 - Preheat your waffle iron according to the manufacturer's instructions.
02 - In a large bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and poppy seeds until well blended.
03 - In another bowl, beat the eggs, then whisk in milk, melted butter, lemon juice, lemon zest, and vanilla extract until thoroughly combined.
04 - Pour the wet ingredients into the dry ingredients and stir gently until just combined. Do not overmix; a few lumps are acceptable.
05 - Lightly grease the waffle iron with butter or nonstick spray. Pour batter onto the preheated iron according to manufacturer's guidelines, close, and cook until golden and crisp. Repeat with remaining batter.
06 - In a small saucepan, combine blueberries, sugar, water, lemon juice, and lemon zest. Bring to a simmer over medium heat, stirring occasionally and gently mashing some berries to release their juices.
07 - For a thicker consistency, dissolve cornstarch in 1 tablespoon cold water, stir into the berry mixture, and simmer for 2 additional minutes until slightly thickened. Remove from heat.
08 - Serve waffles immediately, generously topped with warm blueberry syrup.