Before getting started, rinse and check your mussels to make sure they are all tightly closed.
Cook the bacon in a large skillet over medium high heat until crispy. Remove bacon from the pan using a slotted spoon and reserve. Leave the fat in the pan.
Add shallots and garlic to the bacon fat and cook over medium heat until soft, about 5 minutes.
Add mussels, salt, pepper, cider, and mustard. Cover and bring to a simmer. Cook for about 5 minutes, shaking to move the mussels around the pan. The mussels are done when they are all opened. If you have some mussels that aren’t open, cook them for another minute or two, but discard any that remain closed.
Taste the broth and adjust seasonings, then transfer to a serving platter and garnish with the fresh parsley and cooked bacon. Serve immediately with plenty of your favorite bread.