Recipe: Grub's spinach-mushroom-ginger soup - California Cookbook Step 11. While the stock is simmering, prepare the mushrooms: Heat the oven to 450 degrees. On a rimmed baking sheet, toss the mushrooms with the olive oil and one-half teaspoon of salt and several grinds of pepper, or to taste. Roast the mushrooms until they darken, are aromatic and begin to release their juices, 15 to 20 minutes. Stir the mushrooms about every 5 minutes, so they roast evenly. Step 22. Remove the mushrooms and save any juices that have collected in the pan. When the stock is ready, add the juices from the mushrooms along with the wine, and gently simmer for about 20 minutes to marry the flavors. Taste the broth and adjust the seasoning with additional salt and pepper as desired. Step 33. To plate the soup, in each of 4 bowls, divide the mushrooms, spinach and fresh ginger. Pour the hot broth over and serve immediately.