“Sou bee, sou bee, sou,” Frank Sinatra once sang about this sauce, and the man knew what he was singing about. A union of onion slow cooked in butter and béchamel sauce, soubise sauce brings a rich, luxurious note to the classic chicken breast. Feel free to extend that style and class to the vegetables: mushrooms, Swiss chard, and tomatoes flavored with nutty Parmesan. Start spreading the news, daddy-o, this soubise is a keeper.
Cut mushrooms into ¼" slices. Stem Swiss chard. Cut stems into ¼" slices and coarsely chop leaves. Halve and peel onion. Slice half the onion thinly and mince other half. Mince garlic. Halve grape tomatoes. Pat chicken breasts dry, and season both sides with ½ tsp. salt and ¼ tsp. pepper.
Cook the Chicken
Place a medium pan over medium-high heat. Add 2 tsp. olive oil and chicken breasts to hot pan. Sear until browned, 2-3 minutes per side. Transfer to prepared baking sheet and roast until chicken reaches a minimum internal temperature of 165 degrees, 4-6 minutes. Rest chicken 3 minutes. Reserve pan; no need to wipe clean. While chicken cooks, start vegetables.
Start the Vegetables
Place a large non-stick pan over medium-high heat. Add 1 Tbsp. olive oil and mushrooms to hot pan. Stir occasionally until lightly browned, 3-4 minutes. Add Swiss chard stems, sliced onions, and garlic and cook 2 minutes.
Finish the Vegetables
Add ¼ cup water and Swiss chard leaves and stir occasionally until wilted, 2 minutes. Add grape tomatoes and cook until warmed through, 1 minute. Remove from burner and stir in Parmesan and ¼ tsp. salt.
Make the Sauce
Return pan used to cook chicken to medium heat and add 1 tsp. olive oil and minced onion to hot pan. Stir occasionally until translucent, 2-3 minutes. Add white wine and cook until wine evaporates, 1-2 minutes. Add cream and reduce until thick enough to coat the back of a spoon, 2-3 minutes. Season with ¼ tsp. salt and stir in any accumulated juices from chicken.
Finish the Dish
Slice chicken if desired. Place vegetables on a plate and place chicken next to vegetables. Add sauce on and around chicken.