In a small bowl, whisk together flour and 1 cup milk until well combined. Set aside.: The aroma right away is of clean dairy, and you will notice a smooth, slightly viscous texture as the flour hydrates the milk . This slurry is what prevents lumps when it hits hot liquid, so whisk until it feels silky with no gritty powder. If you still feel gritty bits, let it rest a minute and whisk again; this allows the flour to fully absorb the liquid. A common mistake is adding flour directly to hot liquid, which creates clumps. If lumps form later, you can pass the mixture through a fine sieve or vigorously whisk it while the liquid is simmering.
In a small sauté pan, melt butter and sauté onion and mushrooms together until soft and fragrant. Remove from heat and set aside.: As the butter melts, it will smell rich and slightly nutty, then the onions will become translucent and sweet while the mushrooms release their moisture and concentrate their earthy scent. You should hear a gentle sizzle and see edges of the mushrooms darken slightly, which adds depth. Cook until the onion is soft and the mushrooms have reduced, about the moment they stop giving off water. Overcrowding the pan can steam the mushrooms instead of browning them, so work in a single layer if possible or cook slightly longer to let moisture evaporate.
In a medium saucepan, bring to a boil chicken broth and remaining 1/2 cup of milk together with seasonings; garlic powder, salt, pepper, and parsley. Add milk and flour mixture to saucepan and continue to whisk until the soup thickens. Stir in cooked onions and mushrooms, and remove from heat. Let cool. If you are not using right away, cover and store in the refrigerator.: When the chicken broth and extra milk come to a boil, the steam will carry a savory, rounded fragrance that signals the base is concentrated and flavorful. Boiling briefly helps blend the dried seasonings and bloom their aromas into the liquid, but keep an eye on it to avoid scalding or a milk skin forming. If a skin starts to form, gently skim it away or lower the heat. A mistake here is letting the mixture scorch, especially if using thinner pans; stir occasionally and use moderate heat to maintain a gentle rolling boil rather than a furious one.
Add milk and flour mixture to saucepan and continue to whisk until the soup thickens: Once you pour in the slurry, immediately whisk in steady circles so the starch disperses evenly and begins to swell, turning the broth into a velvety liquid. You will see the texture change from thin to creamy, and small bubbles will form at the edge as it reaches simmer. That glossy sheen is the sign of a properly thickened base. If it becomes too thick, whisk in a splash more broth to loosen it. A common error is stopping whisking, which causes the flour to clump and sink; persistent whisking prevents this and ensures a smooth finish.
Stir in cooked onions and mushrooms, and remove from heat. Let cool. If you are not using right away, cover and store in the refrigerator: When you fold the sautéed vegetables into the hot soup, the steam will lift bright vegetal aromas and you will notice flecks of mushroom throughout the bowl. Removing from heat right after mixing keeps the dairy from separating and preserves the delicate flavors. Letting it cool slightly before covering helps avoid condensation that can water down the soup. If storing, place in an airtight container and refrigerate; reheating gently is key because aggressive boiling can change the texture. A mistake to avoid is reheating too quickly over high heat, which can break the creamy emulsion and produce graininess.
