Which soup has head as its main ingredient?

The soup you are thinking of is likely "tête de veau" (pronounced "tet duh vo"), which is a French dish made with calf's head. It's a traditional and sometimes considered a delicacy, although not as widely eaten today as in the past.

It's important to note that while "tête de veau" is a well-known dish, there might be other regional or less common soups that use head as a main ingredient in other cultures.