Prep the chicken: Pat the chicken dry with paper towels.
Season both sides with salt, pepper, onion powder, and paprika.
Make the sauce base: In a small bowl, whisk together Dijon, optional whole grain mustard, honey, chicken broth, vinegar or lemon juice, and garlic. Taste and adjust honey or mustard as needed.
Heat the pan: Set a large nonstick or stainless skillet over medium to medium-high heat. Add the oil and let it shimmer.
Cook the chicken: Add chicken in a single layer.
Sear 3 to 4 minutes per side until golden and just cooked through. Transfer to a plate and tent loosely with foil.
Deglaze and simmer: Lower heat to medium. Pour in the sauce base.
Scrape up browned bits with a wooden spoon. Simmer 2 to 3 minutes to reduce slightly.
Thicken if desired: For a slightly thicker, glossy sauce, stir in the cornstarch slurry and simmer 30 to 60 seconds until lightly thickened.
Finish the chicken: Return chicken and any juices to the pan. Spoon sauce over the pieces and simmer 1 minute, just to coat.
Garnish and serve: Sprinkle with chopped parsley.
Serve with steamed green beans, roasted broccoli, cauliflower rice, or a small scoop of brown rice.