Halacha Orach Chaim 168
Pizza is essentially bread with cheese on top. Cheese is a meal food. It would therefore stand to reason that a Hamotzei is needed before eating pizza. Reb Shlomo Zalman Aurbach writes that the Bracha on pizza is dependent on the custom of the specific locale. As mentioned earlier, a food that is intended to be eaten as a snack requires a Mezonos instead of a Hamotzei. Based on this, it is necessary to verify the standard custom in the specific location, and the intent of the person eating. If the locals usually eat pizza as a meal, a Hamotzei is required, even if one intends to eat it as a snack.
Knishes theoretically should be similar to the Halacha for pizza, as they are the same concept. Meal items being eaten with bread. However, since there is ample reason to say that they are generally eaten as a snack, it would seem much easier to rely on the opinion that recite a Mezonos rather than Hamotzei.
The Mechaber in 168:15 rules regarding a product that seems very similar to our pancakes, that a Mezonos is said, even if it is eaten as a meal rather than a snack. The reason given is because it does not have the character of bread. Mishna Berura brings the opinion of the Magen Avrohom that rules one should recite a Hamotzei if eaten as a meal. It is interesting that the Chofetz Chaim concludes his comments with the opinion of the Magen Geburim that rules like the Mechaber. In addition, the product discussed by the Mechaber does not seem to have added flavor like our pancakes do.

