Heat olive oil in a pot over medium-high heat. Cook onion until softened but not browned, stirring occasionally, about 5 minutes. Add garlic and saute, stirring, just until fragrant, about 1 minute.
Add wine, and bring to a simmer. Add clams, and stir to combine. Cover and steam until clams have opened, 5 to 7 minutes. Remove from heat. Use a slotted spoon to transfer clams to a rimmed baking sheet to cool, discarding any that do not open. Strain clam broth through a fine sieve, and reserve broth for making stuffing.
When clams are cool enough to handle, remove all meat, and reserve. Remove half of each clam shell, and discard. Arrange the remaining halves on a rimmed baking sheet. Cut each clam into ½-inch pieces; return pieces to shell. (Clams can be prepared to this point up to 1 day ahead. Cover with plastic wrap and refrigerate.)
Preheat oven to 350 degrees F. Dividing evenly, press Herbed Breadcrumb Stuffing into clam shells, smoothing with the back of a spoon. Bake until tops are golden, 25 to 30 minutes. Nestle clams on seaweed, if desired, and serve immediately.