In a large bowl, soak the clams in enough cold water to cover. Add the kosher salt and let the clams sit for 30 minutes to purge any sand. Drain and rinse well.
While the clams are soaking, prepare the rest of the ingredients: Finely chop the garlic, dice both bell peppers and the onion into ¼-inch squares, and thinly slice the ginger. Roughly chop the fermented soybeans. Combine cornstarch with 2 teaspoons of cold water and mix well to form a slurry.
Transfer the clams to a medium saucepan, cover them with cold water, and set them over medium-high heat. Bring them to a simmer, and cook them just until they open, about 5 minutes. Using tongs, transfer the clams to a large bowl, discarding any that remain closed. Strain and reserve ¼ cup of the clam cooking liquid. (Save the rest for soup, if desired.)
Heat the oil in a wok or large skillet over high heat until it is shimmering. Add the black beans, garlic and ginger, and stir-fry until everything is fragrant, 10–15 seconds.
Add the bell pepper and onion to the wok, and stir-fry just until vegetables begin to soften, about 1 minute.
Add the clams along with the reserved ¼ cup of clam liquid and the soy sauce, oyster sauce, sugar and bouillon (if using). Stir-fry until clams are coated and heated through, about 1 minute.
Add the cornstarch slurry (giving it a quick stir), and cook everything, stirring, until the sauce thickens and clings to the clams, about 30 seconds. Taste and adjust the seasoning with additional sugar or soy sauce as needed.
Remove the dish from the heat and let it cool slightly before plating and serving.
