Butterfly the chicken breasts through the center to create four even pieces.
In a shallow dish, combine the flour, garlic powder, salt, and black pepper.
Dredge each chicken breast in the flour mixture, pressing to ensure even coverage, then shake off excess and set aside.
Heat 1 tablespoon butter and 1 tablespoon olive oil in a large pan over medium-high heat.
Once hot, add the chicken breasts and fry until golden and cooked through, about 3-5 minutes per side depending on thickness.
Transfer the chicken to a plate.
Reduce the heat to medium and add the garlic to the pan, sautéing for 1-2 minutes until it just starts to brown.
Pour in the stock, using a wooden spoon to scrape up any browned bits.
Add the remaining 5 tablespoons butter and whisk until melted and combined, forming a slightly cloudy sauce.
Stir in the parsley and season with salt and black pepper to taste.
Let the sauce simmer a few minutes until it thickens slightly, then lower the heat and return the chicken to the pan.
Spoon the sauce over the chicken, serve with extra sauce, and finish with a squeeze of lemon juice if desired.