If you like that stuff in a can, you’ll love my homemade version of the following mushroom soup “concentrate.” Whether making soup or gravy, all you do is reconstitute the concentrate over gentle heat, with enough added liquid (stock, water, milk or light cream) until you’ve reached the desired consistency. When satisfied with the texture, bring up the temperature to piping hot, and you’re good to go. See my notes at the end of this recipe, about making a larger batch for freezing. So, now you can forget the can and truly taste the mushrooms!
If you make the concentrate and freeze it, don’t be concerned if, after thawing, it looks somewhat curdled. This will correct itself, once fully reheated. I would suggest, however, for the most homogenous texture, after thawing, when a recipe suggests that you use the canned soup “straight” (without liquid) you should stir the measured concentrate, over low heat, with a minimum of ¼ cup liquid (or even crème fraiche), until the texture evens out. And, for best color retention, I always add an extra dose of fresh thyme when reheating, since freezing seems to muddy its green color and quiet its delicate flavor.
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Place the dried porcinis in a bowl and pour the boiling water over them. Let the mushrooms reconstitute for 10 to 15 minutes, or until supple. Lift the now supple mushrooms out of the flavorful liquid and retain 1 generous cup of mushrooms and 1 strained cup of the liquid. Chop the mushrooms coarsely and set them aside next to the reserved liquid. Save any remaining reconstituted mushrooms and liquid to use in another recipe.
Measure the cream or milk and pour ¼ cup of it into another small bowl. Stir the cornstarch into the smaller amount of milk and set it next to the reserved porcini liquid, for now. Melt the butter in a 2-quart saucepan and, when hot and bubbling, add the shallots and chopped fresh mushrooms. Cook the vegetables over high heat until the shallots are softened, very fragrant, and the mushrooms give off their liquid. Stir the flour into the wet vegetables and cook, stirring constantly, for 1 minute. Stir the larger amount of milk into the pot along with the porcini liquid and chopped porcinis and bring the mixture to a brisk bubble, over medium heat. Stir in the bouillon paste, if using. Reduce the heat to low and cook for about 5 minutes, until thickened. Give the cornstarch mixture a good stir and pour it into the pot. Stir the mixture as it comes to a simmer, add the thyme and let the soup base cook, stirring frequently, uncovered, for 5 minutes (the soup will become thicker and will take on a slight sheen). Season the soup base with salt and pepper to taste, then remove from the stove and pour into another bowl. Place a sheet of plastic wrap directly on top of the mushroom soup concentrate (to prevent a skin from forming), and let the mixture cool. Refrigerate or freeze for future use. If planning to freeze to use in a recipe that calls for “canned” mushroom soup, divide the concentrate into 1 ¼ cups increments, since that’s the size of a standard can.
To each cup of soup base, add 3/4 cup of any kind of stock or just use water. After reheating, if still too thick, add a bit more liquid (use stock, light cream, milk or water)
The mushroom soup concentrate can be made, cooled and stored in the refrigerator for 5 days, well covered. Alternatively, it can be frozen for 6 months. To thaw, leave in the refrigerator overnight. Reheat gently, but fully, adding as much stock, milk or water, as needed to reach the desired consistency.