Wash the potatoes and cook in boiling water for about 20 minutes. Then rinse with cold water, peel and leave to rest for a while. Peel onions and cut into strips. Peel garlic and cut into fine cubes.
Cut the potatoes into slices. Heat oil in a pan (approx. 28 cm Ø). Fry the potatoes and onion strips in it over high heat for about 5 minutes. Season with salt and pepper. Whisk eggs and season with salt and pepper.
Pour evenly over the pan. Allow to set at reduced heat for 10-15 minutes. (Close the pan with a lid if possible.) Slide the tortilla onto a plate or the flat lid of the pan.
Put the browned side up back into the pan and fry for another 3-5 minutes. Cut the finished tortilla into cake pieces and arrange on plates.