Melt the 3 tbsp butter in a saucepan. Sauté the onion until it softens, about 3-5 minutes. Add the spices (fennel, celery seed, thyme and cayenne) and cook for 1-2 minutes until fragrant.
1 small yellow onion, 3 tbsp unsalted butter, 2 tsp fennel seeds, 1 tsp celery seeds, 1 pinch cayenne pepper
Add the 2 tbsp flour and mix constantly for 4-7 minutes until the color turns medium brown.
2 tbsp flour
Reduce the heat to medium low. Add the vegetable broth, miso and soy sauce and stir until smooth and thickened, a few minutes.
¾ cup vegetable broth, 1 tbsp miso, 1 tsp good-quality soy sauce or tamari
Drizzle in the milk slowly and mix well to ensure it all incorporates well. Stir until somewhat thickened, about 5-7 minutes. It will thicken more as it cools.
¾ cup milk
Remove from the heat. Taste and add salt, pepper and other seasonings as needed.