Grind the fennel seeds coarsely in a spice grinder.
Add the rosemary leaves and ancho chili. Let them sizzle in the oil for about 1 minute. Add the onion, fennel seeds, 1 teaspoon thyme and cook until the onion is softened about 3 to 4 minutes.
Add the beans to the pot and cook a few more minutes, stirring to coat them well in the oil. Add 2 quarts of water and 1 cup of the reserved bean juice. Bring to boil over high heat. Turn the heat to low and place a circle of parchment over the beans to keep them underwater. Simmer, stirring occasionally. After 30 minutes, add 1 ½ teaspoons salt to the beans, and continue cooking at a low simmer until the beans are tender 1 to 1 ½ hours.
Separate the bean mixture from the liquid by straining the soup over a bowl. Put half the bean mixture into a blender with ½ cup of the liquid (you will need to puree the soup in several batches.)
Taste and adjust the seasoning with salt and pepper. Keep in soup warm in pot on the stove.
Heat 2 tablespoons olive oil in a 12 inch skillet over high heat for 1 minute.
Swirl in the remaining 2 tablespoons olive oil and the butter. When the butter melts, scatter the mushrooms into the pan. Season with ¼ teaspoon salt and a pinch of pepper. Cook the mushrooms, stirring occasionally, until they are tender, browned and a little crispy, about 5 minutes. Stir in the remaining 1 teaspoon thyme and remove from heat,
Ladle a cup of hot soup into a warm bowl. Scatter warm mushrooms over the top and add a dollop of mascarpone.
Adopted from Fine Cooking magazine