Preheat oven to 375°F (190°C).
Season chicken breasts with salt and pepper.
In a skillet, heat olive oil over medium heat and sear chicken for 5 minutes on each side.
Remove chicken and set aside. Add sliced onions to the skillet.
Cook onions slowly until caramelized, about 15 minutes.
Return chicken to skillet and top with caramelized onions.
Add Swiss cheese on top of the chicken.
Transfer skillet to oven and bake for 10-15 minutes until cheese is bubbly.
Serve hot, garnished with fresh herbs.