Pat the chicken dry with paper towels. Season with salt and pepper on both sides.
Heat a large skillet over medium-high heat. Add the olive oil and 2 tablespoons of butter. When the butter is foaming, add the chicken. Cook for 4-5 minutes per side, until deeply golden and the chicken reaches 165°F in the thickest part. Cooking time will vary depending on the thickness of the chicken.
Transfer the chicken to a plate and loosely tent with foil.
Add the remaining 3 tablespoons butter to the skillet. Cook until melted, foamy and deep golden brown with a nutty aroma. Watch closely because butter can burn quickly.
Add the sage leaves and fry for about 30-60 seconds, until crisp. Transfer to a plate.
Add the apple wedges to the browned butter. Cook for 1-2 minutes per side until golden around the edges but still firm. Transfer apples to the plate with the chicken.
Add the shallot to the skillet and cook for about 2 minutes, stirring occasionally, until softened.
Reduce heat to medium-low. Add the chicken broth, scraping up the browned bits from the bottom of the skillet. Simmer gently for 3-4 minutes, until slightly reduced.
Stir in the heavy cream and Dijon mustard. Add the cubed Brie and stir frequently over medium-low heat until the Brie is melted, and the sauce is creamy. Do not let the sauce boil. There may be tiny bits left of unmelted Brie.
Stir in the lemon juice. Taste and adjust seasonings as needed.
Return the chicken and its juices to the skillet. Spoon the sauce over the chicken. Arrange the apple slices and crispy sage around the chicken. Warm gently for 1-2 minutes, then serve immediately.