In a small saucepan, whisk together maple syrup, maple extract, brown sugar, oil, garlic, lemon juice, salt and pepper over medium-high heat. Bring to boil, whisking occasionally until sauce begins to thicken, about 5 minutes.
Remove marinade from heat and add thyme. Allow to cool slightly, about 10 minutes.
Place chicken in a gallon Ziplock bag and pour maple marinade over top.
Place in refrigerator for 30 minutes to marinate.
Preheat George Foreman grill by plugging it in, or setting it to 425°F.
Remove chicken, discarding thyme, and place on preheated grill. Grill for 6-10 minutes (depending on thickness), or until chicken reaches internal temperature of 165°F.
Note: if grilling on Indoor/Outdoor grill, grill for 10 minutes per side.
Serve hot with your favorite vegetable and starch.