Preheat the oven to 180°C (350°F), conventional mode.
Whisk the eggs and sugar for about 5 minutes until pale and fluffy, using a stand mixer or hand mixer.
Gradually add the oil while mixing, then incorporate the milk and lemon juice.
Gently fold in the lemon zest, chopped walnuts, and the flour sifted together with baking powder. Mix until just combined.
Using an ice cream scoop, fill the muffin cups about three-quarters full.
Place a walnut half on top of each muffin.
Bake for approximately 25 minutes, or until a skewer inserted into the center comes out clean.
Transfer the muffins to a wire rack and let them cool.
Optional: dust with powdered sugar and enjoy with a cup of coffee.