01 - Combine blueberries, sugar, water, lemon juice, and lemon zest in a small saucepan. Cook over medium heat, stirring occasionally, until berries release juices and mixture thickens, about 8–10 minutes. Remove from heat and set aside to cool slightly.
02 - Preheat waffle iron according to manufacturer's instructions.
03 - Whisk together flour, sugar, poppy seeds, baking powder, baking soda, and salt in a large bowl.
04 - Whisk eggs, milk, melted butter, lemon zest, lemon juice, and vanilla extract in a separate bowl until well combined.
05 - Pour wet ingredients into dry ingredients and gently mix until just combined. Do not overmix; a few lumps are acceptable.
06 - Lightly grease waffle iron. Pour batter onto hot iron (about 1/2–3/4 cup per waffle), spreading evenly. Cook until golden and crisp, about 4–5 minutes.
07 - Serve waffles warm, topped generously with blueberry compote.