I would like that burger with everything, but hold the mustard and tomato.

A ‘burger’ is a very popular food in the United States.  When going to a restaurant, you can tell the person working at the restaurant that you do not want certain things on your burger.  By saying to ‘hold the mustard and tomato’, you are telling them you do not want mustard or tomato on your burger.

我要的漢堡，裡頭夾什麼都可以，就是不要有芥末和馬鈴薯。

英文單字 hold 有多種含義，固然值得查考、細究；而本句中 “… with everything(,) but …” 這樣的表達，也算是常見的英文句構，宜多加揣摩。