Cut lamb into ½-inch cubes (easier if meat is firmed in the freezer for 15 to 45 minutes).
Toast cumin seeds in dry skillet over medium heat, shaking pan occasionally, until fragrant, a minute or 2.
Toss together lamb with cumin, chili, garlic, soy sauce, a large pinch of salt and a healthy grinding of pepper. If you like, cover and refrigerate until ready to cook, up to 24 hours.
When ready to cook, put a tablespoon of oil in a large skillet (ideally, it will hold the lamb in one layer, or nearly so) and turn heat to high. When hot, add lamb. Cook, undisturbed, for about a minute, then stir once or twice to loosen lamb from skillet. Cook another minute, then stir again. Add scallions, if using, and cook, stirring occasionally, until scallions glisten and shrink a bit and the meat is about medium.
If you want a slightly saucier mixture, stir in ¼ cup water and cook another minute. Serve hot over rice, garnished, if you like, with cilantro.