How to Make It
In a large saucepan, cook the bacon strips over moderately high heat until they begin to crisp. Pour off all but 2 tablespoons of the bacon fat. Add the chopped onion, chopped celery and diced potatoes and cook, stirring frequently, until the vegetables begin to soften, about 4 minutes. Add the butter and shiitake mushrooms and cook for 2 minutes. Stir in the flour and cook for 1 minute. Add the chicken stock, white wine, clam juice and heavy cream and bring to a boil. Reduce the heat, cover and simmer the chowder until the potatoes are tender, about 10 minutes.
Meanwhile, in another large sauce-pan, heat the vegetable oil over moderately high heat. Add the mussels, cover and cook, shaking the pot occasionally, until the mussels open, about 3 minutes; discard any unopened mussels. Set a fine sieve over the pot of chowder. Using the lid to hold back the mussels in the pot, pour the mussel liquid through the sieve into the chowder. Simmer the chowder for 1 minute longer. Stir in the chopped parsley and season with hot sauce and salt and pepper. Spoon the mussels into bowls, ladle the chowder on top and serve immediately.