Trim the ends of the zucchini, and slice it on the diagonal ½ inch thick. Set aside. In a large zip top bag combine oil, salt, pepper, and juice from ½ the lemon. Seal bag and shake to combine ingredients. Add zucchini slices to the bag, gently squeeze as much air out of the bag and seal it. Move zucchini around in the bag so it’s all evenly covered with the mixture. Let them sit in the refrigerator for 30 minutes. Place zucchini slices onto a very hot grill, and flip once they get some strong grill marks on them approximately 1 minute or less. Pull them off the grill when they get some good marks on the other side. Taste and season with more lemon juice if necessary.