What about a quality-fast food meal?
Yes, I know, fast food and quality don’t get along too much, it doesn’t sound logic.
However, Five Guys is a good trade-off between a fast food approach, but still you eat quality stuff.
I have tried the one in the city center of Milan, next to the cathedral of the city. Therefore, if you want to go for an informal dinner, while being in the center of Milan, you could go for Five Guys. 
It works like this: you choose as base either a hamburger or a hotdog, and then you add any topping you want. Of course, there is a list of topping where you can choose from. Also, they do milkshakes, I honestly didn’t try it because I generally don’t like them, however I have been impressed that a possible flavor for your milkshake is BACON!
What I tried is a Hot Dog with mayo and grilled onion along with fries. 
In only one word: AMAZING!
Also, I really appreciated the fact that they have an open kitchen, therefore you see everything they prepare and how they prepare it.
I am still wandering what’s the taste of a milkshake with bacon, have you ever tried it?