In the times of the Talmud, people customarily ate garlic raw, without first cooking or grilling it. And so the Gemara establishes that if one eats raw garlic, he recites the ordinary Beracha of "Boreh Peri Ha’adama," whereas if one eats cooked garlic, the garlic is "downgraded" to the Beracha of "She’ha’kol," since cooking garlic was seen as having a detrimental effect on its taste. (Tosafot explain that although cooked garlic is often flavorful, this is due to the ingredients with which it is cooked, but intrinsically, cooking has an adverse effect on the taste.)
The Mishna Berura (Rav Yisrael Meir Kagan, 1839-1933) noted (in Siman 203), however, that people in modern times do not generally eat hard, sharp garlic without first cooking it. Therefore, he writes, nowadays, one who eats raw garlic would recite "She’ha’kol," since he is eating the garlic in an unusual manner. This was also the ruling of Hacham Ovadia Yosef.
As for cooked garlic, the Shulhan Aruch follows the Gemara’s ruling, that one recites "She’ha’kol" due to the detrimental effect of cooking on garlic, and this ruling is cited also by Yalkut Yosef from the Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Jerusalem, 1870-1939). However, the Mishna Berura, as mentioned, established that a food’s status depends on the conventional eating habits of every time and place. This principle should, presumably, affect the status of cooked garlic, just as it affects the status of raw garlic. Today, it is clear that people enjoy cooked garlic, and in fact it is often served as a popular delicacy in hotels and at affairs. Stores even sell readymade cooked garlic, and it is also common for people to grill garlic on a barbeque. Therefore, as this has become the normal way of eating garlic, it would stand to reason that nowadays, the Beracha over cooked garlic would be "Boreh Peri Ha’adama."
Summary: Nowadays, when garlic is commonly eaten cooked or grilled, and is generally not eaten raw, one who eats cooked garlic recites the Beracha of "Boreh Peri Ha’adama," and one who eats raw garlic recites the Beracha of "She’ha’kol."
