Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.

In a small bowl, combine flaxseed with water. Stir and let sit for 10 minutes to gel.

In a large mixing bowl, whisk together flour, sugar, baking powder, baking soda, and salt.

In a separate bowl, whisk together flax eggs, milk, oil, and vanilla extract.

Pour the wet ingredients into the dry and stir gently until just combined. Fold in the blueberries.

Scoop the batter into the muffin tins, filling each cup about ¾ full.

For the streusel, combine flour, sugar, and salt. Cut in the cold butter until the mixture resembles coarse crumbs. Sprinkle evenly over the muffin batter.

Bake for 22–26 minutes, or until the tops are golden and a toothpick comes out clean.

Let cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.