- Heat teriyaki marinade, sesame oil, brown sugar, garlic and ginger over medium heat in a saucepan, stirring regularly until brown sugar dissolves. Allow sauce to simmer for 2 minutes longer. Transfer half of mixture to a medium mixing bowl to cool for 10 minutes and reserve the rest in a small jar for later.
- Slice chicken thighs in half and add to mixing bowl with teriyaki mixture. Toss to coat. Cover and refrigerate for 4 hours or overnight.
- Preheat oven to 400ºF and line a medium baking sheet with parchment paper. Place chicken thigh pieces on baking sheet in an even layer without crowding. Bake 20 minutes.
- Remove baking sheet from oven and cool until you can handle chicken, about 5 minutes. Thinly slice chicken thighs using a sharp knife on a cutting board and return to pan. Drizzle with remaining teriyaki sauce. Turn oven to broil and broil chicken for 10-12 minutes or until thighs are lightly crisped around edges.
- Assemble nachos by scattering hot chicken over tortilla chips. Top with red onion, bell pepper and cilantro.