Heat oven to 400 degrees. In a cast-iron skillet or Dutch oven, melt 1 tablespoon of butter over medium heat. Cook the diced apple until softened, about five minutes. Remove from the skillet and allow to cool for a few minutes.
Mix together the grated cheddar, thyme leaves and cooled apples. Set aside.
Pat the chicken thigh skin dry with a paper towel. Rub 1 teaspoon of salt, the pepper and the poultry season all over the four chicken thighs.
If the skillet is too dry, add a little more butter. Heat the skillet over medium-high heat. When the skillet is hot, add the chicken thighs skin-side down. Cook until the skin browns, about seven minutes. Turn the chicken over and cook for another five minutes. Remove the chicken from the skillet and wipe out the chicken drippings.
When the chicken is cool enough to handle, gently lift the chicken skin and stuff with the cheddar, apple and thyme mixture. Try to keep the skin attached to the chicken, but if it does break, simply secure the skin with toothpicks.
For the creamy garlic sauce, melt the remaining 2 tablespoons of butter over medium heat. Add the flour and continuously whisk for about two minutes. Add garlic and cook for about 30 seconds.
Slowly pour in the chicken stock and heavy cream and add the remaining ¼ teaspoon salt. Whisk until it smooths out. Add the chicken back to the skillet and bring to a simmer.
Place in the oven and bake until the chicken cooks through, the juices run clear and registers 165 degrees, about 15-20 minutes. Remove from oven and stir lemon juice into the cream sauce. If desired, garnish with additional fresh thyme sprigs. Serve over rice. Enjoy!