Start by combining the warm water, sugar, and yeast in a small bowl. Allow it to sit for about 5 minutes, until it becomes frothy.
In a large mixing bowl, whisk together the 3 cups of all-purpose flour and salt.
In another bowl, combine the milk, melted butter, and egg. Add the frothy yeast mixture to this bowl and mix well.
Gradually pour the wet ingredients into the dry ingredients. Stir until just combined; do not overmix. Gently fold in the chilled blueberries and chopped walnuts, being careful not to break the berries.
In a small bowl, mix together the 1/2 cup of all-purpose flour, brown sugar, chopped walnuts, cold butter pieces, and ground cinnamon. Use a fork or your fingers to create a crumbly texture.
Preheat your oven to 375°F (190°C). Grease the muffin tin or line with paper liners. Spoon the batter into each muffin cup, filling them about 2/3 full. Sprinkle the streusel topping generously over each muffin.
Place the muffin tin in the preheated oven and bake for 20-25 minutes or until the tops are golden brown and a toothpick inserted in the center comes out clean.
While the muffins cool, you can prepare a simple glaze by whisking together powdered sugar, lemon juice, and enough milk to reach your desired consistency. Drizzle over the cooled muffins for an extra touch of sweetness.