Add chicken to slow cooker.
Place sliced onions and peppers on top of chicken; add garlic.
Pour chicken broth over peppers, onions, and chicken, then sprinkle dry Italian dressing over everything.
Cover and cook on low 6 hours or on high 3 - 4 hours.
When ready to serve, open hoagie roll, top with chicken, and layer a slice of cheese on each side.
Broil in oven 1 - 2 minutes until lightly browned and cheese has melted.