1. Preheat oven to 350 degrees F. Remove stems from mushrooms. Save stems for another recipe and set caps aside. Chop sunflower seeds in blender or food processor. Set aside.
2. Melt margarine in small saucepan. Toss in onion and garlic, and lightly brown. Add the water and bouillon cube, and bring to a boil to dissolve bouillon. Turn down heat and add sunflower seeds and nutritional yeast.
3. Stir occasionally and bring back to a boil. Add pepper, tamari and paprika. When sauce comes to boil reduce heat and let simmer for 2 or 3 minutes.
4. If you want sauce thicker, add the arrowroot mixture and bring to boil again. Sauce thickens as it cools. Once sauce has cooled, spoon into mushroom caps. Bake for 10-20 minutes.