Add broth and garlic to a deep 4- to 6-quart heavy pot and heat over medium, stirring, until fragrant, about 1 minute.

Stir in tomatoes and half the oregano.

Cook, stirring occasionally, until tomatoes break down - about 5 minutes.

Add clams, and mix well to coat. Cover pot tightly and increase heat to medium high.

Cook, stirring once, until clams open, 5 to 7 minutes. (Discard any clams that have not opened after 7 minutes.)

As clam cook, toss together lemon zest and remaining tablespoon oregano.

Divide clams and pan juices among shallow bowls, then sprinkle with oregano topping.