Season both sides of the chicken with garlic powder, salt, and pepper. Coat the surface evenly so the seasoning forms a flavorful crust as the chicken browns.

Warm the olive oil in a large skillet over medium heat, then lay in the chicken with the smooth side facing down. The oil should give a steady, gentle sizzle when the chicken touches the pan.

Press the chicken lightly with a spatula and sear for 4 minutes. Leave it undisturbed until the underside develops an even golden-brown crust.

Turn the chicken, press lightly again, and cook for 2 minutes. The second side needs color, but the chicken will finish later in the gravy.

Transfer the chicken to a plate. Keep any juices on the plate because they add flavor when the chicken returns to the skillet.

Raise the heat to medium-high, melt the butter, then add the onion and cook for 1 minute before adding the mushrooms. The onion should begin to soften while the mushrooms start releasing their moisture.

Cook the mushrooms until golden, scraping the skillet as they brown. When the liquid has evaporated, the pan will smell richly savory and show browned fond around the mushrooms.

Stir in the flour and cook for 1 minute. The flour should coat the mushrooms and lose its raw, dusty appearance before the broth goes in.

Pour in half of the broth while stirring, then blend in the remaining broth once the mixture is smooth. Scrape the bottom of the skillet so the browned fond dissolves into the developing gravy.

Stir in the sour cream and Dijon mustard until combined. The sauce may look speckled or slightly separated at first, then smooth out as the sour cream warms gently.

Bring the gravy just to a simmer, lower the heat to medium-low, and cook for 3 minutes before seasoning with salt and pepper. It is ready when it flows in a glossy ribbon and resembles pouring cream.

Return the chicken and its collected juices to the skillet, then simmer for 1 minute before taking the pan off the heat. The chicken should be warmed through and lightly coated in the sauce without continued hard bubbling.

Spoon the stroganoff over pasta or egg noodles and finish with parsley or chives if desired. The herbs add a fresh green contrast to the rich mushroom gravy.