Butterfly (or pound to even thickness) the chicken breasts to about ¾ inch thick: slice each breast horizontally almost through and open it. This creates an even thickness so the chicken cooks uniformly — avoiding a dry edge and a raw center.

Pat the chicken dry very well with paper towels and season both sides with 1 teaspoon kosher salt, ½ teaspoon black pepper and the smoked paprika (if using). Dry skin browns; seasoning early helps flavor penetrate the meat.

Heat a large heavy skillet (cast-iron or stainless steel) over medium-high heat until hot. Add 1 tablespoon olive oil and swirl to coat. A properly preheated pan gives an immediate sizzle and promotes Maillard browning — the chemical reaction that builds deep flavor and color.

Add the chicken breasts, presentation side down first (smooth side), and sear undisturbed for 3–4 minutes until a deep golden-brown crust forms. Flip and sear the second side for 2 minutes. Resist the urge to move the chicken; moving prevents that desirable crust from forming.

Lower the heat to medium and add 3 tablespoons (about half) of the butter, the minced garlic, diced onion, and the thyme sprigs to the pan. Spoon melted butter over the chicken (baste) continuously for 1–2 minutes. Basting with butter adds flavor, keeps the surface moist, and the aromatic butter infused with garlic and thyme flavors the meat.

Tilt the pan slightly and use a spoon to baste the chicken repeatedly while cooking for another 3–4 minutes, or until the internal temperature reaches 155–160°F (68–71°C). The butter will help conduct heat and finish the chicken gently without drying it; remove a few degrees before the target final temp because carryover cooking will bring it to 165°F.

Transfer the chicken to a warm plate to rest for 5–7 minutes. Resting allows the juices to redistribute rather than run out when you cut it, keeping the chicken tender and juicy.

While the chicken rests, finish the pan sauce: keep the skillet over medium heat, add the remaining 3 tablespoons butter and let it melt. Scrape the fond (brown bits) off the bottom with a wooden spoon — those bits are concentrated flavor. Add ¼ cup water or low-sodium chicken stock to deglaze and loosen those bits; simmer 1–2 minutes to reduce slightly.

Stir the reduced pan liquid with the sautéed garlic-onion-butter mixture until it becomes a glossy sauce, tasting and adjusting seasoning with a pinch of salt and a grind of pepper. Add chopped parsley if using for freshness. The emulsion of butter and reduced liquid creates a silky sauce that coats the chicken.

Slice the rested chicken on a bias (diagonally) and arrange on warm plates. Spoon the garlic-thyme pan sauce over the chicken, garnish with the thyme sprigs or parsley, and serve immediately. Slicing on the bias creates larger, more elegant pieces and showcases the moist interior.

Optional finish: If you like a little brightness, a quick squeeze of lemon over the plated chicken (not used earlier to avoid curdling the butter in the pan) adds a bright counterpoint to the rich butter.