B737 flight crew reported severe turbulence during cruise flight.

Narrative

Near the country FIR boundary approximately 30 miles southwest of ZZZZ ATC switched us to ZZZ Center and we received a reroute. While in cruise flight and the autopilot engaged; we were copying the ATC clearance. I noticed the ride deteriorating and autopilot and auto throttle becoming unusually aggressive. So I told the Flight Attendants over the PA to be seated immediately as a suspected moderate turbulence was impending. Seconds later; the airspeed became erratic followed by moderate turbulence and loss of altitude. We started the turn immediately given by ATC during the reroute clearance. The airplane was unable to hold altitude; and the increased turbulence caused the autopilot to disengage. The airspeed began to decay quickly with a 30kt loss. The stick shaker activated for a few seconds. I assumed manual control executed the upset recovery maneuver and announced I had the airplane. I suspected we were in an imbedded thunderstorm cell and attempted to change course. I told ZZZ Center we were descending to FL 330 as we were unable to maintain FL 350. In a matter of 20-30 seconds we were out of the turbulence and approaching VMC conditions. The Flight Attendants reported there were no injuries and observed no aircraft damage. We reported the severe turbulence event to ZZZ Center and Dispatch. Lastly this cell did not paint any return even though we were using a downward tilt. An Electronic Log Book (ELB) write up was submitted.