Preheat the oven to 375 degrees F. Grease eight wells of a muffin tin or line them with paper liners.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and poppy seeds until combined. Add the lemon zest and orange zest and whisk to combine.
In a small bowl, combine the egg, milk, vanilla, and oil. Stir to combine. Add the lemon juice and stir. Add the egg mixture to the flour mixture, stirring to combine.
Fill the prepared wells of the muffin tin three-quarters full with the batter. Sprinkle the tops with sugar. Bake for 15 to 20 minutes, rotating the pan after 8 minutes, until the muffins are golden brown on top and spring back when lightly touched in the center.