In a large skillet, heat olive oil over medium-high heat, and brown tenderloin on all sides.
In a small bowl, mix together teriyaki sauce, chicken broth, brown sugar, garlic, red pepper flakes, onion, and black pepper.
Place browned tenderloin in slow cooker and cover with teriyaki mixture.
Cover and cook on low 4 - 5 hours, turning 1 or 2 times to make sure tenderloin cooks evenly.
Remove from slow cooker and let rest 5 minutes before slicing. Drizzle with remaining juices from slow cooker to serve.