Melt the butter in a saucepan over medium-low heat. Add the caraway seeds and cook until fragrant, about 1 minute. Add the mushrooms and cook, stirring, until they begin to wilt, about 5 minutes. Add 1/4 teaspoon salt, and pepper to taste. Add the carrots and broth, cover and bring to a simmer. Uncover and cook until the carrots are tender, 10 to 12 minutes. Season with salt and pepper.
Meanwhile, combine the onion, vinegar and a pinch of salt in a bowl and let marinate while the soup simmers. Toast the bread.
Divide the soup among bowls and top with sour cream and the marinated onion. Serve with the toast.
Photograph by Antonis Achilleos
Courtesy Food Network Magazine