Season chicken breasts with salt and pepper. Heat a skillet over medium-high heat with a little olive oil. Cook chicken for about 10-12 minutes until golden and cooked through. Remove and slice.

In the same skillet, add sliced onions and reduce heat to medium. Sauté for 5-7 minutes until soft and golden brown, scraping up browned bits for extra flavor.

Nestle sliced chicken back into skillet, cover with cheese and cook until cheese is bubbling and melted, about 2-3 minutes.

Preheat oven to broil. Split hoagie rolls and place on a baking sheet. Toast rolls for 2-4 minutes until golden.

Assemble sandwiches by layering cheesy chicken and caramelized onions on rolls. Add optional toppings like mushrooms or jalapeños. Serve immediately.