Preheat oven to 375°F (190°C) or heat skillet over medium.
Combine flaxseed and water; let rest for 5 minutes.
In a bowl, mix mashed lentils, breadcrumbs, onion, garlic, soy sauce, ginger, pepper, and flax egg.
Form mixture into golf-ball-sized balls and place on baking sheet or skillet.
Bake or cook for 15 minutes, turning halfway, until firm and golden.
Warm teriyaki sauce in saucepan.
Toss cooked balls in sauce until coated and heated through.
Serve garnished with sesame seeds or chopped green onions.