B777 Captain reported momentary loss of aircraft control after experiencing severe turbulence at cruise altitude.

Narrative

We were already experiencing continuous Moderate turbulence; the Flight Attendants (FA) were seated for about 1 min when we encountered severe turbulence. Fortunately every one was seated; the cabin was already mostly cleaned up. All passengers had their seat belts on. Aircraft weight was 733;000 lbs. M 0.82; FL300. We experienced momentary loss of aircraft control; significant airspeed fluctuations; including intermittent stick shaker. Several objects in the flight deck hit the ceiling; and significant pull on the seat belts. I came close to hitting my head on the control wheel from a sitting position. No aircraft limitations were exceeded; speed or bank limits.Fortunately; there were no injuries; and the aircraft did not seem to be damaged. The cabin was also intact; no damage other than some spilled drinks and broken glasses. After informing dispatch of our situation; and announcing the encounter on frequency; we determined it was safe to continue to ZZZZ We did consider a divert back to ZZZ; or possibly ZZZ1; but decided it was safe to continue. Dispatches first reaction was very disappointing. He asked us a couple of times if it was really severe turbulence; and asked us to read the manual's definition of severe; like he didn't believe us. Got essentially the same message from Maintenance Control; was it really severe? Yes; unequivocally. In XX years of flying; I have never experienced severe turbulence till this flight. There was never a question about the integrity of the plane; just got the plane really beat up. The radar was on; no weather was observed. Clear air turbulence; unforecast. No suggestions.We did have 1 PIREP from an A330 further N on the ZZZ1 tracks also experienced severe turbulence; but it was not near our route of flight. In the end; I was disappointed of dispatches response; as well as maintenance. It was like they really didn't want us to use the severe turbulence descriptor; because the inspection it triggers. Not the response I was hoping for. Not; is everyone ok; or is the plane ok. But was it really severe?