Preheat oven to 400°C (200°C). Drizzle a 9x12-inch baking dish with olive oil. Keep aside.

Melt butter over medium high heat in a large pan or skillet. Add the sliced onions and season with salt, 1 teaspoon of dried thyme and ground black pepper. Cook for 8-10 minutes or until  soft.

Reduce heat to medium and cook until onions are caramelized ( about 15 minutes), stirring from time to time.

Pour in 1/4  cup of broth and balsamic vinegar and cook until the pan gets dry. The onions should be browned an look jammy.

Add in garlic and cook until fragrant, about 1 minute. Remove from the heat and allow to cool slightly.

Place the chicken breast on a cutting board, put you hand on top of the breast and carefully cut an opening, about 3/4 of the way through the breast. Repeat with the remaining breasts. Season all over and inside with 1 teaspoon thyme, salt and pepper.

Stuff with 2 tablespoons  of french onions, cheeses and seal with toothpicks. 

Transfer the remaining french onion and 1/2 cup of broth to the prepared baking dish. Keep aside.

Heat oil in the same skillet over medium high heat.  Sear the stuffed chicken for 4-5 minutes each side or until golden brown. Transfer to the prepared baking dish.

Top with fresh sage leaves. 

Bake  for 15-20 minutes or until fully cooked. Enjoy!