Prepare the Marinade: In a small mixing bowl, whisk together teriyaki sauce, Dijon mustard, minced garlic, and sesame seeds until smooth.

Marinate the Chicken: Place the chopped chicken breast into a zip-top bag or a shallow container. Pour the marinade over the chicken, seal the bag, and gently massage it to coat all pieces evenly. Refrigerate for at least 2 hours, or up to 24 hours.

Soak the Skewers: If using wooden skewers, soak them in water for at least 30 minutes before grilling.

Preheat the Grill: Preheat your grill to medium-high heat, around 400°F.

Assemble the Kabobs: Alternate threading marinated chicken, fresh pineapple, red bell pepper, mushrooms, and onion onto each skewer.

Grill the Kabobs: Place the assembled skewers on the preheated grill. Grill them for about 11-14 minutes, flipping every 3 minutes.

Let Them Rest: After grilling, let kabobs rest for about 5 minutes.

Garnish and Serve: Garnish with sliced green onions and sesame seeds if desired.