@Brenda
It’s a very popular northern italian sauce. The northern Italians tend to use lots of butter, cream, lard, fatty condiments. The southern Italians tend to be more red, hot and spicy.
Tonnato sauce is typically used with “boring” meats…lean, no flavour, like poached veal cuts, boiled beef tongue and dry chicken breast. I absolutely LOVE tonnato sauce with sliced beef tongue…I used to beg my mom to make it when I was a child. So good…and very traditional italian dish.
Nom nom paleo has a good version of tonnato sauce: