Preheat the oven to 375°F.

Cook Bacon: Put the bacon on a cooling rack over a sheet pan. Transfer to the oven and cook until crisp, about 10 minutes. Let cool on the cooling rack.  Once cool, crumble/chop into small bits and transfer to a medium bowl. Increase oven temperature to 500°F.

Make Topping: Add the breadcrumbs, parsley, thyme, salt and pepper to the bowl with the bacon and mix.  Add enough oil to hold the stuffing together, about 3 Tablespoons.

Roast Mussels: Arrange the mussels on a sheet pan in a single layer. Bake until the shells open, 5-7 minutes.  Remove the mussels from the oven and let cool for about 5 minutes. Turn oven on broil.

Discard any mussels that fail to open.  Gently pull apart the shells of each mussel, holding it over a bowl to capture the juices; strain the juices through a fine strainer and reserve.  Using a knife carefully cut through the muscle that attaches the meat to the shell.  Discard one side of the shell and save the other.  Put the mussel meat with the strained juices in a bowl. Set aside.

Cut Leeks: Cut off the tough green tops of the leeks and the root tentacles. Clean the leeks thoroughly by cutting them in half lengthwise.  Lay each half flat and slice in the center along the length of the leek but this time not through the root.  Fan the leek halves under running water with the root pointed up. Let drain well. Cut off the root and finely chop the leeks.  You may need to wash the leeks again once cut.

Sauté Leeks: In a medium sauté pan, over high heat, melt butter until bubbling.  Add leeks and sauté over high heat until soft, about 3 minutes. Add the garlic and cook for 30 seconds.  Add the cream and cook for 1 minute to let the cream reduce. Add lemon zest, lemon juice and salt. Add mussels and their juices and toss to coat.  Remove from heat.

Assembly: In the base of each mussel shell, spoon a mussel and some of the leek cream mixture.  Put about 2 teaspoons of the breadcrumb topping on top of the mussel in each mussel half-shell.  Mound it to the rim of the shell and pack it tightly.  Place the shells on a sheet pan.

Broil: Broil mussels for 1-2 minutes, until breadcrumbs are golden.  Transfer to a platter.

Serve: Garnish platter with parsley and serve warm.