Rinse lentils thoroughly. Combine lentils and 2 cups water in a pot. Bring to a boil; reduce to a simmer. Cook, checking after 20 minutes, until chewy but still firm, 20 to 30 minutes total. Drain.
Transfer lentils to a frying pan or wok along with garlic, ginger, salt, and pepper. Cook and stir over medium heat until aromatic, 2 to 3 minutes. Add bok choy, 1/2 cup water, black bean sauce, gochujang, soy glaze, and soy sauce. Cook until thickened, 3 to 4 minutes.
Boil a pot of water. Add rice cakes; cook until chewy, 2 to 3 minutes. Drain and add to the lentil mixture.
Blend cashews with a 1/4 cup water in a blender; add to the lentil mixture and cook until flavors combine, 1 to 2 minutes more. Serve with sliced green onion.