Grilled Cumin Lamb Skewers

Name: Grilled Cumin Lamb Skewers
Ingredients: 4 tablespoons whole cumin seeds, divided; 1 teaspoon fennel seeds; 1 1/2 teaspoons sesame seeds, toasted; 2 teaspoons fine gochugaru (Korean chili flakes); Kosher salt; One 3 1/2-pound boneless leg of lamb, trimmed of thick silver skin and cut into 3/4-inch cubes; 1/2 yellow onion, cut into 2-inch pieces; 3 tablespoons neutral oil, such as canola or vegetable, plus more for greasing; 1/4 cup soy sauce; 4 teaspoons cornstarch; 2 cubanelle peppers (about 1/2 pound), halved lengthwise, seeded and cut into 1-1/2-inch-thick pieces