Heat the butter and olive oil in a large skillet (with a lid) over medium heat. Add the onion and sauté for about 5 minutes, until softened.

Season the chicken pieces with poultry seasoning, salt, and pepper on both sides, then increase the heat to medium high and add the chicken to the pan. Cook for 4-5 minutes on the first side, until browned. Flip and cook for another 2-3 minutes on the other side, until browned. Remove the chicken and onions from the pan and keep warm.

Reduce the heat to medium low and add the butter to the same skillet. Once melted, add the flour and whisk to combine. Cook the flour mixture for 1 minute, whisking occasionally, to eliminate the raw flour taste.

Slowly add the chicken broth to the pan and whisk continuously until the mixture is smooth and without any lumps. Sprinkle in the thyme, salt, and pepper. Let the gravy cook over medium-low heat for 2-3 minutes, until slightly thickened.

Return the chicken and onions to the pan and stir well to coat all of the chicken in the gravy. Cover the pan with a lid (or a baking sheet if you don't have one) and cook for 3-4 minutes, until the chicken is cooked through.

Remove the skillet from the heat and stir in the Greek yogurt. Serve the chicken over mashed potatoes with plenty of gravy.