Preheat your oven to 375°F (190°C). Season the chicken breasts with salt and pepper on both sides.

In a large oven-safe skillet, heat the olive oil over medium-high heat. Add the chicken breasts and sear for about 5-7 minutes on each side until golden brown. Remove the chicken from the skillet and set aside.

In the same skillet, add the cipollini onions and sauté for 3-4 minutes until they start to soften. Add the minced garlic and cook for an additional minute until fragrant.

Pour in the balsamic vinegar and bring it to a simmer. Cook for 5-7 minutes until the vinegar has reduced by half.

Stir in the chicken broth and bring the mixture back to a simmer. Slowly add the heavy cream, stirring continuously until the sauce is well combined.

Return the seared chicken breasts to the skillet, spooning some of the sauce over the top. Sprinkle the grated Parmesan cheese and chopped thyme evenly over the chicken.

Transfer the skillet to the preheated oven and bake for about 20-25 minutes, or until the chicken is cooked through and the sauce is bubbly.

Remove the skillet from the oven and let it sit for a few minutes. Serve the chicken topped with the balsamic-glazed cipollini onions and a generous amount of the creamy sauce.