You should consider large chain grocery stores your friend…Aeon, Carrefour, Ole, etc etc. You select what you eat. On that note, look for the Kuai Can ‘fast food’ places where you point and pick what you want…like a panda express with more variety. It’s usually high turnover, fairly fresh and pretty cheap. Lunchtime will be packed and you will eat standing up. Mainly, you see what you are eating, rather than trusting a menu or deciphering some characters you don’t really know yet.
Also, if you make it to more foreigner concentrated areas of town there are maybe more food choices you are keen on. HK is fine for this, lots of salads, organics and foreigner friendly stuff.
Finally, learn some key words to ask for potential allergens such as 花生酱,牛奶and requirements such as 吃素 , 味精,or 清真. Those Hui ethnic places with the green Arabic symbols are halal, in case you are Muslim, like that style of food or are looking for a mosque.