The tonnato sauce can be made the day before, kept covered in the fridge, and will actually become more flavorful. Extra sauce can be served with vegetables as a dip.
If you’re not a fan of mayo, you can use 1/3 cup mayo plus 3 tablespoons olive oil instead — or even just olive oil. The result will be a sauce that’s thinner and not as spreadable, but it will be delicious and closer to the original tonnato sauce served with veal in Italy. We like the sandwichy quality the mayo imparts — and we’re big fans of this sandwich.