Wash potatoes thoroughly and cook in boiling water for 15-20 minutes. In the meantime, clean and wash the cauliflower, carrots and spring onions. Divide the cauliflower into florets. Cut carrots into slices and spring onions into fine rings.
Cook cauliflower and carrots in little boiling salted water for 8-10 minutes. Drain the vegetables and measure 3/8 litres of the vegetable water. Cut ham into strips. Drain potatoes, rinse under cold water and peel.
Mix the potatoes, vegetables, spring onions and ham and place in a greased casserole dish. Heat the fat in a pot and sauté the flour in it. Add vegetable water and milk while stirring, bring to the boil and simmer for about 5 minutes.
In the meantime wash parsley, dab dry and chop. Add 50 g of gouda, thyme and parsley, except for a tablespoon, to the sauce and season with salt, pepper and nutmeg. Pour the sauce over the casserole and sprinkle with the remaining cheese.
Bake in the preheated oven (electric oven: 200 °C / gas: level 3) for approx. 45 minutes. Serve sprinkled with remaining parsley and fresh thyme as desired.