Remove the skin from the potatoes, rinse and cook in salted water for 20 minutes. Cut zucchini lengthwise, hollow out, chop the inside and make with the potatoes for another 5 min. Season zucchini halves with salt, steam in 40 g butter for 5 min on each side until soft, place in a buttered gratin dish and sprinkle with grated Gouda. Press potatoes and zucchini flesh through a press, mix with egg yolks, 50 g ground pine nuts and Parmesan and season. Squeeze potato mixture into zucchini halves and bake for 15 min at 175 °C, sprinkle with remaining pine nuts. For the sauce, sauté diced onion in 20 g butter until translucent, add finely chopped sorrel and whipping cream, uncover for 5 min. Add remaining butter and season with salt and juice of one lemon.
Tip: The flesh of young, small zucchini is much more tender than that of larger ones.