In a medium soup kettle, melt 3 TB. butter. Saute sliced mushrooms and onion until soft. Add minced garlic and saute for 1 more minute. Melt last TB. of butter in with the mushroom mixture and add 3 TB. of flour. Cook, stirring gently to coat, for 1 more minute.
Add chicken broth, then simmer and stir until bubbly and thickened. Add half & half, parsley (if using), and salt and pepper. Heat on medium-low to heat through. Do not boil once you have added the half & half or it will separate. Check and adjust salt and pepper at the end as some broths and stocks are saltier than others.
Ladle into bowls and add a pinch of freshly grated nutmeg, if desired.
For Swiss Mushroom Soup: Butter thick slices of French bread and sprinkle with garlic powder. Place under broiler to toast adding thick slices of Swiss cheese during the last minute. Top hot bowls of soup with cheesy garlic toast.
For Mushroom Spinach Soup: Add a few handfuls of fresh, chopped spinach (stems removed) during the last 5 minutes of cooking.
For Bacon Mushroom Soup: Cook 1 1/2 " pieces of Maple Peppered bacon until just crisp. Drain and add to bowls of soup with chunky garlic croutons.