Serve juicy stuffed chicken in only an hour.
In a pot of boiling salted water, cook sliced Swiss chard stems 2 minutes. Transfer to a bowl.
Add leaves to pot and cook 1 minute. Drain, squeeze out water, chop and add to bowl. In same pot, heat 2 Tbsp. olive oil over medium heat. Add red onion and cook until tender, 7 minutes.
Add chard and season with 1/2 tsp. salt. Cover and set aside. Preheat oven to 400°. Starting from thinner side, cut a pocket in every chicken breast half. In each, place a 3/4-ounce piece of Gruyère and 1/2 Tbsp. each parsley and thyme. Use 2 toothpicks to seal each breast shut. Season with 1/4 tsp. each salt and black pepper and 1 tsp. Dijon mustard. In a large, ovenproof skillet, heat 1 Tbsp. olive oil and cook chicken until golden, 4 minutes. Flip, transfer skillet to oven and bake until chicken is just cooked, 12 minutes.
Transfer chicken to a plate. Add white wine to skillet and simmer until reduced by half. Add reserved chard and 1 Tbsp. each Dijon and unsalted butter. Divide chard among 4 plates, top with chicken and accumulated juices and serve.
Get the full recipe: Gruyère-Stuffed Chicken with Chard Recipe