Heat butter in a large saucepan over medium-high heat. Add onions and garlic and cook until soft. Add remaining ingredients and cook for 15 minutes. Place mixture in a blender and blend until smooth. Return to the saucepan and cook an additional 15 to 20 minutes, or until thickened. Preheat grill. Brush chicken with vegetable oil and season with salt and pepper to taste. Grill chicken for 6 to 7 minutes on each side or until golden brown and cooked through. Remove chicken from the grill and brush with the sauce.