Scrub clams with a brush, and rinse well to remove sand. Place in a large kettle with ½ cup water, and steam until shells open, about 15 minutes. Remove clams from shells, and strain juice. Set clam juice aside until ready to use.
Brown garlic in oil about 3 minutes. Add tomatoes, salt, pepper, and parsley. Simmer about 20 to 25 minutes, stirring occasionally. Add clam juice and clams. Continue cooking 1 minute longer. Pour over toast and serve immediately.