Scrub mussels under running water; discard any that do not close. Cut off beards. In large pot, combine water, half of garlic and 3/4 teaspoon pepper. Bring to a boil. Add mussels and steam on medium until they open, about 7-10 minutes. With slotted spoon, transfer mussels from broth onto 2 rimmed baking sheets. Break off and discard top halves of shells and any unopened shells.
Heat olive oil in medium skillet. Saute remaining garlic and onion until soft. Add parsley and lemon juice. Combine with cracker crumbs and Parmesan cheese. Sprinkle over mussels. Bake at 400 F about 5 minutes or until crumbs are lightly brown. Serve warm with lemon wedges.