1. Pat the chicken breasts dry with paper towels. Season both sides generously with salt and black pepper. This step is crucial for developing a good crust and ensuring the chicken is flavorful throughout.

2. Heat the olive oil and 1 tablespoon of butter in a large skillet over medium-high heat. Once the butter is melted and shimmering, add the seasoned chicken breasts to the pan. Sear for 5-7 minutes per side, or until golden brown and cooked through. The internal temperature should reach 165 F (74 C). Remove the chicken from the skillet and set aside on a plate, tented loosely with foil to keep warm.

3. Reduce the heat to medium. Add the remaining 1 tablespoon of butter to the same skillet. Add the finely chopped onion and cook for 3-4 minutes, stirring occasionally, until softened and translucent. Then, add the minced garlic and cook for another minute until fragrant, being careful not to burn it.

4. Pour in the chicken broth, scraping up any browned bits from the bottom of the pan with a wooden spoon. This step, called deglazing, adds incredible depth of flavor to your sauce. Bring the broth to a simmer and let it reduce slightly for about 2-3 minutes.

5. Stir in the heavy cream, fresh lemon juice, and chopped fresh thyme leaves. Continue to simmer the sauce gently for 3-5 minutes, stirring occasionally, until it thickens slightly to your desired consistency. Taste the sauce and adjust seasoning with additional salt and pepper if needed.

6. Return the cooked chicken breasts to the skillet, nestling them into the creamy sauce. Spoon some of the sauce over the chicken to coat. Let it warm through for 1-2 minutes. Garnish with fresh chopped parsley, if desired, before serving immediately.