Here's the soup that inspired the Campbell's classic. Cream of mushroom soup was wildly popular in the early 20th century, and if you try your hand at making it from scratch, you'll see that the real stuff has a freshness that could never come from a can.
Melt 3 tablespoons butter in a 4- to 5-quart heavy-bottomed pot over medium-high heat. Add mushrooms, onion, salt, and pepper, and cook, stirring often, until golden brown, about 15 minutes. Transfer to a bowl and set aside.
Reduce heat to medium and melt remaining 3 tablespoons butter. Add flour and cook, stirring, until glossy, about 3 minutes. Slowly drizzle in chicken broth and milk (or half-and-half), whisking as you go. Increase heat to a simmer and cook until mixture thickens, whisking often, about 5 minutes.
Reduce heat to medium-low, stir in the reserved mushroom mixture, and cook 10 minutes, stirring occasionally. Remove from heat and add the sour cream.
Serve garnished with chopped fresh parsley.