Duck and Foie Gras Ravioli in Mushroom Fumet with Shiitake Caps and Chervil

This rich ravioli, filled with chicken breast and duck and foie gras, also contains salty, dense duck prosciutto, which gives a pleasing chewiness to the pasta. You can make this prosciutto at home (pack a duck breast in salt for 24 hours in the refrigerator, then rinse the salt off and set on a rack or hang to dry at room temperature for 7 days) or buy it from a purveyor; we use D’Artagnan. The broth uses well-cooked mushrooms for its base—white button mushrooms are an underrated ingredien