Rinse the white cabbage and cut it into quarters. This makes it easier to slice into strips
Slice the white cabbage into strips about a couple of centimeters wide.
Pour water into a pot (about 5 liters capacity), add salt and bring to a boil.
Add the cabbage to the boiling water and cook until tender / al dente. This takes about 15 minutes.
Lift the cabbage out of the cooking water. Do not discard the cooking water.
Now the flour needs to be "cooked off": Find a saucepan and add butter to it.
Let the butter melt over medium-high heat
Add the flour to the melted butter and stir well to form a roux.
Then reduce the heat and now add some of the cooking water from the cabbage to the saucepan while whisking with a whisk. Continue to slowly add a little cooking water at a time, as a sauce gradually forms. You will need about 3 deciliters of cooking water in total. I used 2 deciliters of cooking water from the cabbage and 1 deciliter from the ham I served with it. If you're not making ham, just use 3 deciliters of the water from the cabbage. Remember to wait before adding more liquid until the sauce is fully combined and homogeneous.
Now thin with milk while continuing to whisk.
Let it all simmer for 5 - 7 minutes. This ensures that the sauce doesn't taste of flour.
Then add the white cabbage to the sauce.
Finish by adding thyme and seasoning with freshly grated nutmeg and salt and pepper.