Air carrier Captain reported encountering severe turbulence and aircraft upset. Flight regained control and continued to destination safely.

Narrative

On Day 0 while operating flight from ZZZ1 to ZZZ we experienced what felt like some kind of environmental condition which caused the aircraft to begin to flutter and then enter a slight nose down attitude with a right bank angle putting the aircraft into an upset. During the time of event we were cruising in smooth air at flight level 360 flying at an assigned speed of .79 Mach. There were no reports of turbulence in the area nor were we in proximity to another aircraft to the best of our knowledge and ATC's. The event onset relatively quickly it started as rumbling feeling; the best way I could describe it would be what felt like wake turbulence. The turbulence grew rapidly and the plane entered a right bank angle with the autopilot engaged. ROLL AUTHORITY ROLL AUTHORITY was annunciated by the aircraft to which I responded with UPSET" and followed proper procedures (disconnected autopilot and auto throttles PUSH; ROLL; THRUST; STABILIZE ) to recover to aircraft to a safe flight condition. Contact was made with ZZZ Center and we received a lower altitude; then we re-automated the aircraft. We were informed by ATC that there was no one ahead of us to have caused a wake turbulence event; so we reported to ATC that we believed we had encountered severe turbulence. I established contact with the flight attendants to confirm if everyone was okay; no injuries were reported and they inspected the cabin to ensure all passengers were secure. As normal flight had been restored and there were no injuries in the cabin we elected not to request priority handling and continued to the destination safely."