Heat the olive oil in your large skillet over medium heat. Add the finely chopped onion and sauté until it becomes soft and translucent, which usually takes about 5 minutes. Then, add the minced garlic and cook for another minute until fragrant.
Pat the chicken breasts dry and season them generously with salt and pepper on both sides. Place the chicken in the hot skillet and sear for about 5 minutes per side. You are looking for a beautiful golden-brown crust, which adds a ton of flavor.
Carefully pour the Irish whiskey into the skillet. Use a wooden spoon to scrape up any browned bits stuck to the bottom of the pan—this is where the flavor is! Let the whiskey simmer and reduce for a couple of minutes until the alcohol smell has cooked off.
Pour in the chicken broth and heavy cream, stirring to combine everything. Bring the sauce to a gentle simmer and let it cook for about 10 minutes. The sauce will begin to thicken into a luscious, velvety consistency perfect for coating the chicken.
Stir in the fresh thyme leaves. Taste the sauce and adjust the seasoning with more salt and pepper if needed. Serve the chicken immediately, generously spooning the whiskey cream sauce over the top.