Melt 1 TBSP of the butter in a dutch oven or large pot over medium-high heat. Add the mushrooms, onions, ½ tsp salt and ¼ tsp pepper. Cook, stirring occasionally, until most moisture has evaporated, about 8 to 10 minutes. Stir in garlic and cook 30 seconds or until garlic is fragrant.
Stir in the flour until well combined, about 1 minute. Slowly stir in the chicken broth little by little until well combined. Bring to a boil, reduce heat to low and simmer 3 to 5 minutes.
Heat the oven to 350 degrees F. Stir the sherry and stock into the soup and set aside.
Mix the remaining ingredients into a 9 x13 baking dish. Pour the soup over top.
Cut a piece of parchment paper or wax paper to fit over the liquid inside the pan and let it float on top.
Cover tightly with foil. Bale in the preheated oven for 2 hours.
Remove from oven and allow to cool, with the foil still on, for 15 minutes.
Garnish with more green onions if you wish.
