1. Cover the dried mushrooms in hot water and soak for at least 4 hours, or overnight. Drain the mushrooms, squeezing to extract as much liquid as possible. Strain the soaking liquid through a fine mesh strainer and reserve. Coarsely chop the mushrooms, discarding any tough or woody parts and set aside.
2. Coarsely chop the fresh mushrooms and, working in batches, sauté them in butter over medium high heat in until flecked with brown; transfer to a bowl, toss with lemon juice, and set aside.
3. Add the port or wine, scraping the bottom of the pan, and simmer for a minute or two to burn off the alcohol.
4. Meanwhile, over medium-high heat, sauté the garlic and onion in olive oil until translucent. Season with thyme, salt, pepper, and cayenne. 
5. Working in batches, process the onion mixture and fresh and dried mushrooms in a food processor or blender until finely chopped. 
6. In a 3-quart saucepan, combine the onions and mushrooms and all the beef broth and bring to a slow simmer; cook over low heat for at least 30 minutes.
7. Add the half-and-half and heat; whisk the egg yolks with a bit of the hot soup to temper, and add the yolks to the pot. Cook and stir until smooth and slightly thickened. Adjust lemon juice and seasoning to taste.

Serve at once.

Note: Soup may be prepared through step six up to three days in advance. Just before serving, heat the soup through and finish with cream and egg yolks.