Why did He say "u'Meshol El Beis ha'Meri"?
Radak: The Mashal is, the pot is Yerushalayim. You will make this Mashal - unlike they say [that Yerushalayim will be for you like a pot; the meat does not leave until it finishes cooking].
Malbim: The Mashal is for Beis ha'Meri, for they were Mechadesh the Mashal. They said that the fortified wall of Yerushalayim will protect us, just like a pot protects meat from being burned from the fire.
What is "Sefos ha'Sir"?
Rashi: Where the oven is set down, this is called Sefisah, like Chazal said 'room for Sefisah of two pots' (Shabbos 38b). Sefisah is before putting anything inside. It is an example for Yerushalayim - Hi Lo Sihyeh Lachem l'Sir v'Atem Tihyu v'Sochah l'Vasar" (11:11); Nebuchadnetzar will put it on the fire. Menachem says that it is an expression of putting.
Radak: First a person sets down a pot 1 . Afterwards, he pours water in it and then puts the meat in it, and kindles the fire under it, until the meat cooks. Putting the pot down is a Mashal for Melech Bavel coming close.
Malbim: They stuck the oven [to what it is placed on] well, so the fire will not come underneath, only to the sides lest it burn due to the great fire. I.e. they will arrange the wall with all strength.
Radak: Here it says "Bo" (masculine); in most of the Parashah it is feminine, for the Nimshal (Yerushalayim), or because [a Sir] is also called Kalachas, which is feminine.
What is the significance of pouring water in it?
Rashi: These are needs of the siege.
Radak: It is a Mashal that it will not catch fire quickly. Water delays the meat from cooking quickly. When there is no water, the meat roasts quickly; when there is water, it cooks slowly. The siege was prolonged, from Tzidkiyah's ninth year until his 11th, so people will slowly perish in Yerushalayim, from hunger, plague and the sword.
Malbim: They said not to build houses near the wall (11:3), i.e. the meat should not be near the wall without an intermediary, for then it burns before it cooks. They trusted in the pot i.e. the wall, and the water (their weapons), that the [enemy's] fire will not burn them and they will be in Yerushalayim until the cooking finishes (their death).