For the soup I always use the soup greens that can be bought and add 3-5 potatoes, depending on the size. But I leave out the leek because it pulls threads when cooked.
Cut the vegetables into pieces of your choice except for the onion. Peel the onion but leave it whole. Boil everything in a saucepan with 2-2 ½ liters of water until soft.
In the meantime, finely chop some fresh parsley. Make a light brown roux from the flour and oil. It shouldn`t be too light, otherwise the soup will taste like flour. Fish out the onion and stir the roux into the soup. Be careful when adding, as it can splash!
Let the soup simmer for a few more minutes, then season with salt and sprinkle with the finely chopped parsley.