Wash the zucchini, quarter lengthwise and cut into slices. Clean, wash and halve or quarter the mushrooms. Clean, wash and cut spring onions into rings. Heat 2 tablespoons of oil in a pan with lid (24 cm Ø). Fry zucchini and mushrooms for about 5 minutes, turning frequently.
Peel and wash the potatoes and cut them into large pieces. Season zucchini and mushrooms with salt and pepper, remove. Heat 2 tablespoons of oil in the frying fat. Fry the potatoes for about 15 minutes, turning them in between. Sauté spring onions briefly. Add zucchini and mushrooms
Whisk eggs and milk. Season with salt, pepper and marjoram. Pour over potatoes and vegetables. Cover and allow to stand for 15-20 minutes at low heat.
Turn the tortilla out of the pan, cut into pieces and garnish. Serve with herb quark or aïoli (stir 2 chopped garlic cloves into the "pure" homemade mayonnaise from page 40)