fancy: to "fancy" in this context means to desire or have a liking for. for example, "i fancy some onion rings" means "i would like to have some onion rings."
mustard: a condiment made from the seeds of the mustard plant, often used as a topping or flavoring for various dishes. in this context, it is mentioned as a preference for the double cheeseburger.
sounds delicious: this phrase is an expression used to convey that something sounds very tasty or appealing.
a light dressing: "dressing" refers to a sauce or mixture used to add flavor to salads. "a light dressing" implies a small or minimal amount of sauce to accompany the chicken wrap.
come with: this phrase is used to inquire whether a particular item is included with another. for example, "does that come with salad?" means "is salad included with the chicken wrap?"
that's enough: rachel is expressing that the amount of salad that comes with her dish is sufficient; she doesn't need any more.
filling: "filling" in this context means satisfying or satiating. when steve says, "it's not filling enough for me," he means the initial order is not enough to satisfy his hunger.
enjoy your meal: a polite expression used to wish someone a pleasant and enjoyable dining experience.