Finely chop the onion and garlic, add salt and spices, mix and set aside.
In a bowl, combine the egg and room-temperature butter, and whisk into a smooth mixture.
Add the flour with the baking powder and knead a firm dough by hand.
Press the dough out by hand and roll it out thin with a rolling pin, to 3-5 mm.
Finally, spread the onion mixture over the top, pressing it into the dough. Cut the shortbread sheet into squares.
Bake at 180 °C (355 °F) for 30-35 minutes, until golden and fragrant with onion. Let the finished cookies cool - ready to eat. Enjoy!