1/4 cup minced onion
1/2 cup butter, divided
3 tablespoons all-purpose flour
6 cups chicken stock
2 sprigs flat-leaf parsley leaves, plus more for garnish
Pinch of dried thyme
1 1/2 pounds mixed mushrooms, I used shiitake, cremini, and portobello, trimmed and thinly sliced
Juice of 1/4 lemon
2 large egg yolks
1/2 cup heavy cream
Plain whole yogurt (dollop as a garnish, optional)
1. Cook onions in 4 tablespoons butter in a large stockpot over medium heat until soft, 4 to 5 minutes. Stir in flour and cook over low heat for 4 minutes without browning. Add the stock, blending it completely with the flour. Add the parsley and thyme, and simmer for 20 minutes.
2. In a large skillet, heat remaining 4 tablespoons butter over medium heat. Add the cremini and portobello mushrooms and season with salt. Cook until softened, and then add the shiitake mushrooms. Season with salt and lemon juice and cook until mushrooms are deep golden brown, 8 to 10 minutes.
3. Reheat stock mixture to a simmer. Whisk the egg yolks and cream until blended. While whisking, slowly add a cupful of the hot soup into the cream mixture and add back to the soup and heat to just below a simmer (if the soup simmers, the yolks will curdle). Add the cooked mushroom mixture and stir to combine. Serve garnished with parsley and plain yogurt.