Cut the onion into cubes and mince the garlic very small. Also slice the leek (thin slices) and cut the potato, peeled, into cubes.
Cover the bottom of a pan with oil and place the onion and garlic. Season with salt and cook over low heat for 5 minutes. Add the minced garlic, stir well and sauté for 10 minutes, stirring occasionally to prevent sticking.
Add the potatoes and season with more salt. Add all the water and let it cook for 20 minutes (the potatoes should be tender). Then add the cream and mix well.
Put everything in a blender and blend well, until you get a homogeneous cream.
Return to the pan and cook for 20 minutes. Taste and adjust seasoning if necessary.