Bring ingredients to room temperature and preheat oven: Remove chicken from the fridge 15–20 minutes before cooking so it cooks more evenly (cold chicken tightens and can undercook in the center). Preheat the oven to 425°F (220°C) for the roast potatoes.

Season the chicken: Pat the chicken breasts dry with paper towels to remove surface moisture (this ensures a better sear). Season both sides generously with 1 tsp kosher salt and 1/2 tsp freshly ground black pepper. Tuck the chopped thyme and rosemary into any folds or press onto the surface—herbs close to the meat flavors the exterior while searing.

Prep the potatoes and beans: Toss the halved baby potatoes with 2 tbsp olive oil, 1/2 tsp salt, 1/4 tsp pepper and a pinch of red pepper flakes if using. Spread them cut-side down on a rimmed baking sheet so they get maximum browning. Trim green beans and toss with the remaining 2 tbsp olive oil and a pinch of salt; set aside to roast briefly later. Roasting potatoes at high heat produces a crispy exterior and a fluffy interior.

Roast the potatoes: Place the potatoes in the preheated oven and roast for 25–30 minutes, flipping once halfway through. Start the potatoes first because they need the longest single cook time; oven roasting concentrates their starches and gives caramelized edges.

Sear the chicken: While potatoes begin roasting, heat a large heavy skillet (cast-iron or stainless steel) over medium-high heat until shimmering. Add 1 tbsp olive oil and 1 tbsp butter—fat combination gives a high smoke point plus flavor. When the butter foams but before it browns, add the chicken, presentation side down. Sear undisturbed for 4–5 minutes until a deep golden-brown crust forms; this Maillard reaction develops savory flavor and a pleasant texture. Flip and sear the other side 3 minutes.

Finish chicken in the oven: Transfer the skillet (if oven-safe) to the 425°F oven for 6–8 minutes (or move chicken to a baking dish) until an instant-read thermometer reads 155–160°F (68–71°C). We remove at 155–160°F because carryover cooking will raise the temperature to 165°F while resting. Rest the chicken on a plate covered loosely with foil for 6–8 minutes to allow juices to redistribute.

Roast the green beans: In the last 8–10 minutes of the potatoes' cook time, add the tossed green beans to the same baking sheet (or a separate one) and roast together. This timing gives the beans a tender-crisp bite and allows them to pick up some of the potato caramelization without overcooking.

Make the pan sauce base (build flavor): Wipe the skillet clean of excess fat with a paper towel but leave the fond (browned bits). Return the skillet to medium heat and add the minced shallot. Sauté 1–2 minutes until translucent—this softens it and tames sharpness. Add the minced garlic and cook 20–30 seconds until fragrant; garlic cooks fast and can burn, so watch closely.

Deglaze and reduce: Pour in the white wine (or 1/3 cup chicken stock if avoiding alcohol) to deglaze the pan, scraping the fond with a wooden spoon. Allow the wine to reduce by about half (2–3 minutes)—reducing concentrates flavor and cooks off alcohol. Add the chicken stock and bring to a gentle simmer to continue flavor concentration.

Finish the sauce: Whisk in 2 tbsp unsalted butter, the Dijon mustard, and the lemon juice off-heat, swirling to emulsify into a glossy sauce. Taste and adjust seasoning with salt and pepper. The butter and mustard help thicken and round the sauce while lemon brightens and balances richness.

Return chicken to the pan to coat: Add the rested chicken breasts back into the skillet for 1–2 minutes, spooning sauce over them. This reheating and saucing step warms the meat and infuses the surface with flavor without overcooking the interior.

Plate and garnish: Divide the roasted potatoes and green beans among 4 plates. Place a chicken breast on each, spoon 2–3 tablespoons of the lemon-garlic pan sauce over the top, and finish with a sprinkle of chopped parsley and an optional extra squeeze of lemon. Serve immediately to enjoy the contrast between the crispy potato, tender chicken, and silky sauce.