Precook the lasagne in boiling salted water for about 5 minutes. Remove with a skimmer, rinse cold and drain well.
Clean and wash the vegetables. Cut the zucchini in half lengthwise. Cut tomatoes and zucchini into thin slices. Wash thyme, shake dry, pluck off leaves.
Heat the oil in a large frying pan. Fry the zucchini for about 5 minutes while turning. Season with salt, pepper and lemon juice. Stir in sour cream and 2/3 thyme.
Cut the ham slices in half. Place 1 pasta plate on a baking tray lined with baking paper and place 1 slice of ham on top. Cover one half of the pasta plate with some zucchini and tomato, fold the other half over.
Cover remaining pasta plates in the same way until all ingredients are used up.
Finely crumble the toast between your hands. Grate parmesan. Mix with crumbs and the rest of the thyme. Sprinkle on pasta plates. Spread butter in flakes on top. Bake in a hot oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for 15-20 minutes.