In a large pot melt the butter and add the onion over medium-low heat. Cook, stirring until soft but not browned, about 5 minutes.
Add the flour and stir about 2 minutes, to cook out.
Add the water, bouillon and cabbage and set heat to medium. Bring to a boil, then cover and simmer over medium-low heat until vegetables are tender, about 20 minutes.
Adjust salt and pepper, if needed.
Notes
Variations
Dairy-free soup: Substitute olive oil for butter.
Make it gluten-free by replacing all-purpose flour with a gluten-free multipurpose flour, like Cup4Cup.
Add more vegetables such as carrots, celery, or potatoes.
Broth options: Swap water and bouillon for vegetable broth, or use chicken bone broth to increase the protein, but you may need to adjust salt.
Make it heartier: Add white beans, chopped chicken breasts or sliced sausage to the broth.