Instructions

• If using 2 large chicken breasts, slice each breast horizontally into 2 even cutlets so you have four thinner pieces; if using 4 small breasts, leave them whole. Pat the chicken dry with paper towels and season both sides generously with salt and pepper.
• Place a deep 12-inch skillet with a lid over medium-high heat. Add 1 tablespoon olive oil and heat until shimmering.
• Add the chicken to the hot skillet in a single layer (do not overcrowd). Cook undisturbed until the first side is browned, about 3 to 4 minutes. Flip and brown the second side, about 3 to 4 minutes more.
• Remove the browned chicken from the skillet to a plate and set aside.
• Reduce the heat to medium. Add 1 tablespoon butter to the same skillet and let it melt.
• Add 4 garlic cloves (pressed or minced) and 1 sweet onion (halved from root to tip and sliced into 1/4‑inch slices). Stir and scrape the bottom of the pan with a wooden spoon to loosen any browned bits. Cook, stirring occasionally, until the onions are wilted and beginning to soften, 5 to 8 minutes.
• Add 2 (14.5‑ounce) cans diced tomatoes (with their juices), 1/4 cup balsamic vinegar, 1 teaspoon granulated sugar (optional), 1 teaspoon dried oregano, 1 teaspoon dried basil, 1 teaspoon dried, crushed rosemary, and 1/2 teaspoon ground thyme. Stir to combine and bring the mixture to a simmer.
• Return the chicken to the skillet, nestling each piece into the simmering sauce. Cover the skillet and cook until the chicken reaches 165°F in the thickest part, about 5 to 10 minutes (time will vary with thickness).
• Transfer the chicken to a clean plate or cutting board, tent loosely with foil, and let it rest while you finish the sauce.
• Uncover the skillet and simmer the sauce over medium heat until the onions reach your desired tenderness and the sauce reduces to a thicker consistency, about 3 to 5 minutes. Taste and add salt and pepper if needed.
• Spoon the sauce over the rested chicken and serve.