The Poskim discuss whether it is permitted to place bread on the Blech in order for it to become toast. A number of Poskim rule that it is prohibited, each giving a different reason. The first one to address this issues wat the Shoel U’Meshiv (Rabbi Yosef Shaul Nathansohn, 1808–1875, Poland). Based on the Rambam (Shabbat 9:3), who writes that hardening a soft material is a Tolada (corollary) of Mevashel (cooking), he concludes that it prohibited. Hacham Bension has a different understanding why it forbidden. He says that any enhancement of a food is considered Bishul. Just as there are opinions that "Yesh Bishul Ahar Afiya"-one may not cook a food after it has been baked-since the baking enhances the food, so too, the toasting adds a new level to the bread and is forbidden. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Rav Pa’alim, prohibits making toast because of "Makeh B’Patish"- adding the finishing touch to the bread. The Menuhat Ahaba (Rabbi Moshe Halevi, Israel, 1961-2001-vol. 2) also prohibits making toast. He argues that the general principle of "En Bishul Ahar Bishul"-there is no cooking after cooking- only applies when the second cooking does not add to the previous cooking. But here, regarding toast, the normal way to make toast is to apply a second baking to the already baked bread, and therefore the bread is considered raw vis-ŕ-vis the second baking and is prohibited.
Hacham Ovadia (Hazon Ovadia vol. 4) and Hacham David (Halacha Berura 318) disagree and hold that it is permissible to make toast. They refute the arguments of the Poskim who prohibit it. Regarding the Shoel U’Meshiv’s reading of the Rambam, they argue and say the Rambam was only talking about the specific cases he mentioned-i.e. hardening metal, wood, wax and tar-not food. Regarding the Ben Ish Hai’s application of "Makeh B’Patish," the Be'ur Halacha (Rav Yisrael Meir Kagan of Radin, 1839-1933) clearly holds that it does not apply to food. They also reject Hacham Bension’s comparison to "Yesh Bishul Ahar Afiya," since that refers to adding a different process of cooking after baking, whereas here it is just a continuation of the same baking process. They also disagree with the Menuhat Ahaba’s Hidush that the bread is considered raw. It is too extreme. According to that, anytime a second cooking enhances an already cooked food, it would be prohibited.
The conclusion is that it is permitted to make toast, based on the general principle of "En Afiya Ahar Afiya"-There is no problem baking an already baked food.
It is permitted to make toast on Shabbat by placing bread on the Blech or hotplate.