Wash the asparagus. Peel white asparagus. Cut the woody ends off both types of asparagus. Cut the asparagus separately into 3-4 cm long pieces. Wash the potatoes thoroughly and drain well.
Whisk the egg and 75 g cream well. Season well with salt, pepper and nutmeg. Finely slice the potatoes and place them in 3 layers on a baking tray (25 cm x 30 cm) covered with baking paper. Sprinkle each layer evenly with the egg cream.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 30 minutes. In the meantime, cook white asparagus in 1 litre of boiling salted water with 1 teaspoon of sugar for about 8 minutes. Add the green asparagus about 3 minutes before the end of the cooking time.
Then pour the asparagus into a sieve, collecting the asparagus water. Measure out 500 ml asparagus water. Melt butter in a saucepan, dust with flour, sweat and gradually bring to the boil while stirring with the asparagus water and 100 g whipped cream. Simmer for 1 minute while stirring. Season to taste with salt, pepper, sugar and lemon juice.
Add the asparagus to the sauce and heat up again. Wash the thyme, shake dry, pluck leaves from the stalks. Remove potatoes from the oven. Sprinkle with thyme and cut into very large cake pieces. Arrange a piece of cake, some asparagus in sauce and cottage ham on plates.