How can I mount a DS18B20 tempreature sensor to a large heatsink?
That is one poorly defined data-sheet, there is no mention of thermal issue on what is a temperature probe!
 However, there is this link to the thermal aspects of these one wire devices which give you thermal conduction coefficients.  This takes you through self heating effects.  Your device is actually listed.


Now all they need to do is give you the thermal mass so you can estimate the response time ...

I suspect that you'll just have to measure that.  But if you are attaching to a large thermal mass as it is, that mass will dominate.

You can get thermally conductive epoxies made specifically for this task, and bond the flat side down to your plate.