Preheat a grill or grill pan over medium heat. Lightly grease with nonstick spray.
Season the chicken on both sides with chicken seasoning rub. Use more if needed.
In the same pan melt the butter on medium low heat. Saute the garlic and onion until soft. Add the broth, thyme, and lemon juice; bring to a simmer to build flavor.
Grill the chicken on both sides getting nice char marks to an internal temperature of 160 degrees using a thermometer. Then set aside on a cutting board.
Season the sauce with salt and pepper to taste. Adjust the lemon as needed. Slowly stir in the cornstarch mixture a little at a time until the liquid becomes a thickened smooth consistency. (If the sauce is too thick, thin it out with a little more broth, if too thin, continue simmering until it reduces to a creamy consistency.)
Spoon the sauce over the chicken. Serve with bread. Garnish with thyme.
Notes
Pro tips: Butterfly the chicken: Lay the breasts down on a cutting board, using a sharp knife parallel to the cutting board slice the breasts through the middle (carefully) all the way in half.