Combine 1 1/2 cups water and thyme in a heavy skillet with lid; bring to a boil over high heat. Add clams and reduce heat to medium; cover skillet and cook until clams open, about 5 minutes (discard any clams that do not open). Using a slotted spoon, transfer clams to a bowl. Strain juices through a fine-mesh sieve into a bowl; set aside. Remove about half of the clams from shells; discard shells.

In a large skillet, heat 2 tablespoons oil over medium-high heat; add tomatoes and pinch salt; cook, shaking pan back and forth once or twice, just until slightly softened, about 2 minutes; set aside.

Put broth in a medium saucepan; heat to a gentle simmer.

Meanwhile, in a large saucepan, heat 2 tablespoons of the remaining oil over medium heat. Add shallot and cook, stirring frequently with a long-handled wooden spoon, until softened, about 3 minutes.

Add rice, increase heat to medium-high, and cook, stirring frequently, until rice is translucent, about 4 minutes. Add wine; cook, stirring frequently, until wine is mostly absorbed.

Add about 1/2 cup of the reserved clam juices. Reduce heat to medium and cook, stirring constantly, until juices are mostly absorbed. Continue adding juices in 1/2 cupfuls, stirring constantly, and allowing each addition to mostly absorb before adding the next, until all of the juices have been used, then, using simmering broth, continue in the same manner until rice is tender, yet firm to the bite (you will have broth leftover).

Stir in remaining 2 tablespoons oil, shelled clams and parsley. Add broth to moisten, if desired. Divide risotto among serving bowls and top with the clams in their shells, and tomatoes. Sprinkle with pepper and a little salt, and drizzle with oil.