Grill apple slices about 1 minute on each side to soften and create grill marks. At the same time, on a grill safe pan, toast pepitas until fragrant. Set grilled apples and toasted pepitas aside. In a large skillet, melt butter. Add onion and celery, then saute about 5 minutes until softened. Stir in cooked brown rice. Add thyme, salt and pepper. Dice grilled apples and add to the pilaf mixture. Stir in toasted pepitas. Keep warm while chicken is grilling. Combine pumpkin pie spice, sage, salt and pepper in a small bowl. Sprinkle over both sides of the chicken breasts. Grill 8-10 minutes on each side until chicken is cooked through and no longer pink. Serve grilled pumpkin spice chicken with brown rice pilaf.