Heat oven to 350º. 

 

Place potato in a small saucepan; cover with several inches of cold water and add generous pinch salt. Bring water to a boil, then cook until potato is tender, about 15 minutes. Drain and mash with a fork.

 

Place clams in a medium saucepan with 2 tablespoons of water; cover and cook over high heat until clams open, about 8 minutes (discard any that do not open); remove from heat. Drain clams, and, when cool enough to handle, remove meat from shells; discard shells.

 

In a shallow bowl, stir together breadcrumbs, 2 tablespoons parsley, marjoram and thyme; set aside. In the bowl of a food processor, pulse fish until finely chopped but not pasty; transfer to a large bowl. Add mashed potato, 1/2 teaspoon salt and generous pinch pepper to fish; stir to combine. Shape mixture into 24 (1 1/4-inch) balls. Roll fish balls in breadcrumb mixture to coat.

 

Place 6 fish balls in the center of 1 parchment square; add 1/4 of the clams, 1/4 of the tomatoes, 1/4 of the olives and generous pinch salt; drizzle with 1 tablespoon oil. Brush edges of parchment with egg white, then fold paper over filling to form a triangle; crimp edges of paper tightly to seal and enclose filling completely. Place packet on a rimmed baking sheet; prepare 3 more packets in same manner. 

 

In a medium saucepan, heat remaining tablespoon oil over medium-high heat; add rice and cook, stirring frequently, until lightly toasted, about 1 minute. Add broth and 1 cup water; bring liquid to a boil, then reduce to a gentle simmer, cover and cook 20 minutes. 

 

Meanwhile, bake packets until fish balls are cooked through, about 20 minutes. 

 

When rice is ready, remove pan from heat and let stand, covered, 5 minutes. Add remaining tablespoon of parsley to rice and fluff with a fork. Serve fish ball packets with rice.