How to supply a very accurate ADC reference voltage to STM32?
You use an external "reference". You can use anything from a few-penny LM431 to much more sophisticated references, depending on  your requirements for accuracy and stability as well as noise 

LM4040 is a commonly used shunt reference for moderate accuracy and low cost, but there are many others, do a parametric search based on your actual requirements. They come in two basic types- series and shunt. The series type is like a more accurate and stable type of voltage regulator, the shunt type is like a zener diode that requires some series resistance to establish a bias current. 

In some cases, for the best accuracy, you may need to buffer the reference output in order to drive the ADC reference input.