I am a firm believer in roasting or sautéing vegetables to coax out their flavors, and this soup is a prime example of that step. Slowly sautéing the onions (as any good Hungarian mother would!) makes the soup richer, the flavors deeper, and the taste a little sweeter. Sautéing the mushrooms correctly also adds another layer of flavor. Yes, good cooking takes patience, but the final result is worth it.
3/4 cup plus 2 Tbsp (1 3/4 sticks) unsalted butter (divided) 4 medium yellow onions, finely chopped (about 4 cups)
1 1/2 pounds mushrooms, thinly sliced (about 6 cups) 2 1/2 tsp kosher salt (divided)
1 tsp freshly ground black pepper (divided) 2 Tbsp paprika
2 to 4 tsp chopped fresh dill (divided) 5 cups chicken or vegetable stock or canned low-sodium broth
1/2 cup all-purpose flour 2 cups milk 2 tsp lemon juice (about 1/2 lemon)
1 cup sour cream 1/4 cup chopped fresh Italian (flat-leaf) parsley (optional)
- Heat a medium (6- to 8-quart), heavy stockpot over medium-high heat. When very hot, add 4 tablespoons of butter (don’t wait for the butter to melt) and the onions, and sauté, stirring now and then, until very soft and just beginning to color, 10 to 15 minutes. (You might need to turn down the heat a bit if the onions start to color before they get soft.) Remove the onions in to a bowl and set aside.
- Increase the heat to high. When the pot is very hot, add another 4 tablespoons of butter (don’t wait for the butter to melt) and immediately add the mushrooms (this way the mushrooms will sear before releasing their moisture, so they’ll sauté better). Season with 1/2 teaspoon salt and 1/4 teaspoon pepper, stir to combine and sauté the mushrooms until they start to brown. (If they release their moisture, continue to cook over high heat until the liquid evaporates and the mushrooms start to sauté again.) Continue to cook over high heat, stirring occasionally, until the mushrooms are evenly golden, 6 to 8 minutes. (Lower the heat if necessary to prevent scorching.)
- Lower the heat to medium (if you haven’t already), add the paprika, and cook for about 1 minute while stirring. Return the onions to the pot. Add 2 teaspoons of the dill and 3 cups of the stock while stirring to combine. Cover and simmer over the low heat for 15 minutes.
- Meanwhile, in a 3- to 4-quart saucepan, melt the remaining 6 tablespoons of butter over medium heat and whisk in the flour. Cook, stirring occasionally, for about 3 to 5 minutes, until the mixture is light gold and looks like fine, wet sand.
- Slowly whisk the milk into the roux, a little at a time, allowing the roux to absorb the liquid after each addition. Add the remaining 2 cups of stock and whisk again. Bring to a boil, then immediately reduce the heat to a gentle simmer and cook, whisking occasionally, for 10 minutes.
- Pour the thickened milk into the pot with the mushrooms, being sure to scrape up the browned bits from the bottom of the pan. (If it’s at all lumpy, pour it through a sieve rather than directly into the pot, and use a rubber spatula to push through every last drop.) Stir well to combine.
- Cover and simmer for 10 to 15 minutes, stirring occasionally. Just before serving, add the remaining 2 teaspoons of salt and 3/4 teaspoon of pepper, the lemon juice, sour cream, and another 2 teaspoons of dill, if desired.
- Ladle into bowls and serve garnished with chopped fresh parsley, if desired.