Do you really trust into the Hechsher of Jerusalem restaurant ? I have experienced it quite a few times and the incident keeps on repeating itself:
A restaurant or cafe in Jerusalem is officially under the Kashrut supervision of the local Rabbanut ("Mehadrin" or "Mehadrin min ha'Mehadrin"). Great ! Everything seems to be kosher. But is this really the case ?
Can it be the case when I see Palestinian kitchen staff turning on the stove and baking rolls and Croissants ? Not only warming up the goods but starting the entire baking process. Suddenly the baked goods are not Mehadrin anymore but Akum, as a Gentile is not allowed to start a cooking / baking process. Meaning, turning on the oven. Furthermore, it is questionable whether a Jewish staff member is even stirring a spoon after a Gentile turned on the stove in order to cook a meal.
The Mashgiach from the Rabbanut wouldn't agree to a Gentile worker turning on the oven in order to bake buns. However, there doesn't seem to be such a strict supervision either. Many times I have seen a Mashgiach showing up, walking around for a few minutes and this was it.
The Badatz Hechsherim are under stricter supervision and so are the one's of the Sephardi "Beit Yosef". A Belzer Mashgiach once told me that I shouldn't go to places where there is a Rabbanut supervision but, in case their is not Belz or Badatz Edah Charedit, I should refer a place where there is Beit Yosef.
Beit Yosef, Badatz Edah Charedit as well as Badatz Belz carry out a much stricter and more reliable supervision and their Mashgichim mostly show up on a daily basis. The ingredients are being checked thoroughly and so is the cleanliness of the restaurant / cafe. However, those business owners with a Badatz Heshsher most also pay much more to the Edah, Belz or Beit Yosef. "The costs for a Hechsher would be far too exaggerated", thus the claim of the business owners. Moreover, if you as a business owner decide having a Belzer Hechsher, the Mashgiach only wants you to buy Belz ingredients and, Chas veChalilah, nothing with an Edah Hechsher. The same is happening as soon as an Edah Mashgiach spots out ingredients with a Belzer Hechsher. Let alone a Rabbanut or Beit Yosef Hechsher which is totally unexceptable to the chassidic Badatzim.
Today, a Hechsher doesn't necessarily stand for kosher food but rather for politics. The Belzer consider their Hechsher as the only kosher one but force their clients to buy Belzer ingredients with Belzer Hechsherim. Thus, the Belzer also support their own food businesses. The Edah works the same way and the Rabbanut doesn't keep a strict supervision but the Mashgiach shows up from time to time. The Beit Yosef is strict but rejected by the Ashkenazim as it is a Sephardi Hechsher.
Plenty of restaurants are sick of the internal Kashrut politics and paying horrendous amounts of money for supervision. I know restaurants in Jerusalem which are kosher but work without a Hechsher. These businesses are closed on Shabbat, use kosher ingredients and the place is clean. The only difference is that they don't have an official Hechsher. I actually do eat at such a place sometimes which suits me more than going to a certain cafe chain where a Palestinian is turning on the stove. And this chain has a Rabbanut Hechsher.
Haredim won't go into any restaurant without a decent Hechsher but even if there is one, you still need to investigate. The best solution, however, is: Eating at home. And this, by the way, I actually enjoy most !
Jerusalem Kosher News on restaurant owner challenging the Rabbanut