How to fix the protein content of my minestrone?

I noticed you didn't mention beans, which are fairly common in minestrone.

Cannellini beans are most typical, but you could experiment with others (garbanzo, fava beans, etc.)

I use to add old parmigiano crusts (cleaned with a brush) and/or a 10cm cube of prosciutto, just before the soup starts to boil.

The parmesan left on the crust will melt and blend to the soup, while the crusts will become tender and flavory.
The fat in the prosciutto will melt blend to the soup too, while the prosciutto block will cook in a delicious way.

For both additions, remember to use less salt in the soup, because both prosciutto and parmigiano are salty.

My mother used to add these ingredients sometimes, both in minestrone and bean soup, I guess she learned it from her mother, and I keep doing it too.

Or, if you're in a hurry or don't want to add any of those ingredients, you could, at serving, add cubed swiss cheese to the bowls, and/or a couple of spoons of fresh grated parmesan.

It will be difficult to add protein while maintaining authenticity, as minestrone is a vegetable soup, and your main sources of protein are meat and fish. You might add some roasted, shredded chicken to keep a rustic feel, or perhaps some tofu cubes. Or you could grate plenty of fresh parmesan on top.

Of course, there's nothing wrong with having one or two meals without much protein either.