Remove dark green tops from scallions and thinly slice; set aside for serving. Thinly slice remaining white and pale-green parts. Heat oil in a large saucepan over medium-high. Cook white and pale-green parts of scallions, onion, and garlic, stirring often, until softened and starting to brown, about 4 minutes. Add gochujang and cook, stirring often and reducing heat as needed to prevent burning, until very fragrant and slightly darkened in color, about 2 minutes. Add kimchi, gochugaru, and sugar and cook, stirring, until kimchi is slightly softened and translucent, about 2 minutes. Add lentils, broth, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt and stir to combine. Bring stew to a boil, then reduce heat and cover. Simmer, adding a splash of water if needed to loosen, until lentils are just barely tender, 20–25 minutes.