B737 Captain reported a severe turbulence encounter resulted in a temporary loss of control during cruise flight. One Flight Attendant was not wearing a seat belt and received minor injuries from the event.

Narrative

Aircraft X from ZZZ - ZZZ1. On the Pacific Track Alpha; R463. Shortly after passing the critical point; CP; after ZZZZZ; at FL360; we approached the northern edge of a SIGMET area of forecasted moderate CAT turbulence from FL300 - FL360. Radar was on; showing scattered green return after adjusting tilt for best display. Verbal PIREPs from other aircraft indicated light chop to occasional moderate chop was the overall consensus. None were getting a smooth ride; but the higher altitudes reported better. Weather chart showed no turbulence on forecast above FL360 though the information was several hours old from before departure. Previously asked by ATC; via CPDLC; 'When can we accept FL370?' and we said; 'At this time;' but did not receive clearance. We were mostly above the cloud layer; but occasionally in and out of the tops. Seat belt sign was on. Got a moderate jolt and said; 'Flight attendants; take your jump seats;' on the PA. Requested FL370 due to weather. Received clearance via CPDLC and climbed to FL370. Ride slightly better and now above the cloud layer. Continuous light chop ensued; with a couple moderate bumps; for the next 20 minutes. During a period of only intermittent light chop; I called back on the service interphone to check on the flight attendants. I told them possible weather still ahead; based on a green radar return; and even though it was relatively smooth; I wanted them to remain in their jump-seat until we passed the weather; approximately 20 minutes. Roughly 5 minutes later; just after we passed over the ZZZZZ1 fix; airspeed and altitude began to fluctuate. This was quickly followed by a pitch up; autopilot disconnect; high speed clacker and stick shaker; and moderate or worse jolts of turbulence and Saint Elmo's fire on the forward windscreens. The copilot called 'upset' and hand-flew the airplane. He worked to keep the airspeed between high and low speed limits while allowing the altitude to vary and keep G-loading to a minimum; as I called out altitude and climbing/descending. Altitude variation of +/- 1;000 ft.; airspeed variation of +/-30 kt. Bank angles of 15 degrees. Incident lasted approximately 2 minutes. Reengaged automation. Checked on flight attendants and passengers. One Flight Attendant in the aft galley had been thrown to the ceiling and then back on the floor; as well as side to side. She did not have her seat belt on at the time. All other flight attendants and passengers were buckled in and OK.Reported severe turbulence to other aircraft over VHF. Aircraft Y at FL380 approximately 20 miles ahead said they had nothing worse than a moderate bump. Started phone patch with Dispatch on HF to give PIREP; initiate a maintenance report with Maintenance; and gave injury assessment of the Flight Attendant. Later on; after a failed SATCOM call attempt; made another phone patch to include MEDLINK; to determine medical capabilities at destination; and give updates on Flight Attendant condition - after time had passed to assess her condition further. Copilot flew the aircraft and handled ATC communications as we flew into radar reception range. Flight Attendant said she had no broken bones or sprains; just soreness and possible bruising. She felt 'tossed around and wringed out' but fully functional and capable to continue to destination; as well as fly the return leg back to ZZZ. We were given the okay to continue to ZZZ1 from MEDLINK; and they arranged for paramedics to meet the aircraft at destination. Copilot flew RNP approach to landing in ZZZ1 on Runway XX. Paramedics met the plane at the gate and escorted the Flight Attendant off the airplane. Maintenance also met the aircraft; and after a verbal pass down; began their inspection. After all passengers had disembarked; copilot and I went to check the on Flight Attendant and speak to the paramedics. After concluding their evaluation; they determined her fit to fly. Outbound Captain was also in attendance for this evaluation. After this; Icalled MEDLINK to follow up with the EMS determination. I then called the Chief Pilot prior to leaving the airport.