Be sure the clams or mussels are well scrubbed. Mussels should be de-bearded.
In a large, heavy saucepan, heat the oil.
Add the garlic and wine, stir, then add the clams or mussels. Stir, then cover and cook over medium heat until the shells open, about 8 minutes.
Transfer the clams or mussels to 2 warm bowls.
Strain the juices from the pan through a very fine strainer or a strainer that is lined with a piece of muslin or cheesecloth and pour the juices over the clams or mussels.
Scatter the rosemary over each serving and sprinkle with lemon juice.
Serve at once with crusty bread.