Place ground beef, scallions, teriyaki sauce, sesame seeds, breadcrumbs, egg, and water in a large mixing bowl and stir well to combine. Cover and refrigerate for 20 minutes to allow the mixture to firm up.

To make the marinade, whisk the teriyaki sauce and sesame oil in a small bowl.

Clean and dry the mushrooms, discarding the stems, and place them in another mixing bowl with the pepper and onion. Add half the marinade to the vegetables and toss well to coat. Set aside.

Once the meat mixture is chilled, form 48 small meatballs. Place them

in a baking pan and pour the remaining marinade over the top, turning gently to coat. Place the pan in the refrigerator and chill for another 20 minutes to make the meatballs firm and easy to skewer.

Meanwhile, soak wooden skewers in water for at least 20 minutes to prevent them from burning during cooking.

Preheat grill to medium-high and spray the grates well with oil.

Thread the chilled meatballs and vegetables onto the skewers, alternating them.

Grill for 10–12 minutes, turning occasionally, until the meatballs are

browned and cooked through and the vegetables are crisp-tender.

Preheat oven to 425 degrees Fahrenheit (220 degrees Celsius). Line a baking sheet with aluminum foil.

Thread the chilled meatballs and vegetables onto the soaked skewers and

arrange them on the prepared baking sheet. Bake for 18–20 minutes, turning the skewers halfway through, until the meatballs are cooked through and the vegetables are lightly caramelized.

Drizzle with teriyaki sauce and sprinkle with sesame seeds.