Rinse and wipe the dirt from the outside of 1 ½ lbs cremini mushrooms. Cut off the ends then slice each mushroom into pieces.
Add ¼ cup salted butter and 2 cups chopped white onion to a large pot over medium-high heat. Melt the butter and cook for 2 minutes, until the onion turns translucent. Add 3 minced garlic cloves and mix them in. Cook for 30 seconds.
Add 1 ½ lbs sliced cremini mushrooms, 2 Tbsp soy sauce and 1 Tbsp fresh thyme. Mix and cook for 3 minutes.
Sprinkle ¼ cup all-purpose flour over the mushrooms and mix together.
Next, pour in 4 cups vegetable broth (1 carton of broth), 1 tsp salt and ½ tsp ground black pepper. Bring to a boil, then reduce heat to let it simmer for 10 minutes.
Stir 1 cup heavy cream into the soup. Let this simmer for 2 minutes. Remove from heat and serve.