How to Make It
Preheat oven to 500°.
Arrange mussels in a single layer in a 13 x 9-inch baking dish. Bake at 500° for 8 minutes or until shells open; discard any unopened shells. Remove mussels and cooking liquid from dish. Strain cooking liquid through a sieve over a bowl; discard solids. Remove meat from mussels. Reserve 24 shell halves. Discard remaining shells. Arrange reserved mussel shells on a jelly roll pan; place 1 mussel in each shell.
Heat a small saucepan coated with cooking spray over medium heat. Add leek; cook 3 minutes or until tender, stirring frequently. Add garlic; cook 30 seconds, stirring frequently. Stir in cooking liquid, clam juice, and half-and-half; bring to a boil. Cook until reduced to 1/4 cup (about 3 minutes). Stir in rind and juice.
Place bread in a food processor, and process until fine crumbs measure 3/4 cup. Combine the breadcrumbs, parsley, thyme, oregano, oil, pepper, and bacon. Spoon 1/2 teaspoon sauce over each mussel. Lightly pack 2 teaspoons breadcrumb mixture into each shell. Broil 1 1/2 minutes or until golden.