In Sant'Ambrogio, right next to the famous market, you find a wonderful little enoteca which is great for lunch. The place has several tables outside which makes it perfect for warmer days, but you can also eat inside.
A short and simple menu is listed in the window. Three primi and three secondi. You can choose just one thing or you can go for the "Menu completo" which gives you primo, secondo, water and coffee for only 10 euros! I don't think you find it much cheaper than this in Florence. Last time I went I had a risotto with leeks which was amazing and my mum had penne with tomatoes and asparagus. Also very delicious. The service is very friendly and the atmosphere is calm and relaxed. A perfect place for a lunch or a glass of wine before dinner.
Via de' Macci 92r. T. +39 3388846455