Rinse the potatoes and cook in boiling salt water for about 20 minutes. Drain the potatoes, peel and let cool.
Mix the mayonnaise, yogurt and herbs. Season with salt and pepper. Peel the onions and cut into rings.
Slice the cooled potatoes. Heat the oil in a large skillet. Saute the potatoes until crispy, turning occasionally. Slide the potatoes to the edges of the pan and melt the butter. Add the onions and saute with the potatoes. Season with salt.
Serve the potatoes with the herb dip and garnished with lettuce and marjoram.