Let's cook.
Pat chicken breasts dry and season both sides with salt and pepper.
Heat olive oil in a large oven-safe skillet over medium-high heat. Sear chicken breasts for 3-4 minutes per side until golden brown. Remove chicken from skillet and set aside.
Add sliced onions to the same skillet, reducing heat to medium-low. Cook, stirring occasionally, for 10-12 minutes until softened and caramelized. Add minced garlic and dried thyme, cooking for 1 minute more until fragrant.
Pour in beef broth, scraping up any browned bits from the bottom of the skillet. Bring to a simmer and cook for 2-3 minutes until slightly reduced.
Stir in heavy cream. Return chicken breasts to the skillet, nestling them into the onion mixture.
Sprinkle shredded Gruyere cheese over the chicken and onions.
Transfer the skillet to a preheated oven at 375°F (190°C) and bake for 15-20 minutes, or until the chicken is cooked through (internal temperature reaches 165°F or 74°C) and the cheese is bubbly and golden.
Remove from oven and let rest for 5 minutes before serving.
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