In a dry pan, roast the walnuts over medium heat for 1 to 2 minutes.

In a large bowl, mix the flour, baking powder, baking soda, and salt. In a medium bowl, place the sugar, lemon zest, butter, eggs, vanilla, melted butter, and oil. Mix again.

Create a well in the center of the dry ingredients. Pour in the wet ingredients and mix gently; do not overbeat. Fold in the walnuts and blueberries. Pour the batter into a greased and floured 4 x 8-inch (10 x 20 cm) loaf pan. Bake at 350°F (180°C) for 55 to 60 minutes or until a toothpick inserted in the center comes out clean. Without unmolding, let cool on a wire rack for 20 minutes; unmold, then let cool to room temperature.

In a small saucepan, mix 3 tbsp (45 ml) of sugar and lemon juice. Bring to a boil over high heat and cook until the sugar is completely dissolved. Brush the still-warm cake with this mixture.