In a large saucepan over medium-high heat, melt butter until foaming. Add mushrooms and cook until they have released most of their liquid, about 5 to 7 minutes.: The moment the butter foams, you should smell a warm, nutty scent that prepares the pan for the mushrooms . The sound will be a gentle sizzle as the surface heats. This step primes the pan so the mushrooms brown evenly rather than sticking. If the butter smokes, your heat is too high, which will make it bitter. Lower the heat slightly and skim any dark solids if they form. Getting the pan temperature right ensures the next stage develops deep flavor.
Stir in chicken broth, onion powder, and garlic powder and bring to a simmer. In a small bowl, whisk together milk and flour.: As the mushrooms warm they will soften, shrink, and exhale moisture, creating a gentle simmer in the pan. Visually, they will turn from pale to a richer tan, and their edges will wrinkle. You will notice a concentrated, earthy aroma filling the air. Stir occasionally so they cook evenly and so any fond loosens from the pan, which adds taste to the soup. Avoid crowding the pan, because steaming prevents that desirable reduction. If they are still watery after 7 minutes, keep cooking in short intervals until the liquid mostly evaporates.
Stir in milk mixture and cook until soup has thickened, about 1 minute. Season to taste with salt and pepper.: When you add the chicken broth , the pan will click as the liquid meets the warm surface, and the scent will shift toward savory richness. The powdered onion and powdered garlic dissolve quickly, distributing aroma through the liquid. Bring the mixture to a gentle simmer so flavors marry; vigorous boiling can flatten the aromatics. A common mistake is rushing to the thickening step before the flavors have mingled, which results in a less cohesive taste.
For un-condensed soup: Add 2 cups water to make 2 servings of soup, 2 cups each (stir over medium heat until heated through).: Whisking the milk and flour into a smooth slurry prevents lumps and yields a velvety texture. The mixture should be uniform and slightly fluid. Smell it and note a neutral, creamy scent that signals it is ready to blend. If you skip this mixing and add dry flour directly, you will risk gritty clumps. If the slurry seems too thick, add a splash more milk to loosen it, but do not thin it excessively or the soup will take longer to thicken.
Stir in milk mixture and cook until soup has thickened, about 1 minute: As you pour the slurry into the simmering broth, you will hear a soft change in sound as the liquid integrates. Within moments the soup will gain body and a glossy sheen. The aroma becomes creamier, and the spoon should coat the surface with a ribbon of soup that slowly settles. This brief cooking sets the flour to release its thickening power without developing a raw taste. Watch closely because overcooking can make the texture pasty. If it thickens too quickly, lower the heat and whisk gently to smooth it out.
Season to taste with salt and pepper: This is where the soup finds its voice, as the bright bite of freshly ground black pepper and measured salt draw out the umami from the mushrooms . Add seasoning a little at a time and taste between additions, because the chicken broth may have already contributed salt. The aroma should lift and the flavors should feel balanced across the palate. A frequent error is adding too much salt up front; it is best to fine tune at the end when the soup has reached its final texture.
For un-condensed soup: Add 2 cups water to make 2 servings of soup, 2 cups each (stir over medium heat until heated through) : When you dilute the condensed base with water, the soup will loosen and steam as it warms. Stirring over medium heat helps integrate the additional liquid without breaking the silky texture. You will see gentle bubbling and smell a lighter, more delicate aroma. Heat until just hot through, then taste and adjust seasoning because dilution can mute flavor. Overheating after dilution can separate creaminess and reduce the pleasant mouthfeel, so warm just until ready to serve.
