PC12 Captain reported temporary loss of aircraft control in severe turbulence during cruise.

Narrative

At XA51 on Day 0; Aircraft X had a severe turbulence encounter overhead ZZZ; FL210; 170 KIAS; heading 310; due to Aircraft X PIC poor assessment of convective turbulence threat resulting in inadequate lateral separation from convective turbulence. While deviating laterally upwind for convective weather; aircraft experienced approximately 5 seconds of severe turbulence; resulting in an uncommanded climb to approximately 21;150 ft and an estimated force of -0.5 to -1.0 Gs. Aircraft X PIC disconnected autopilot and autothrottles; selected idle power; and returned aircraft to cleared flight path. When stabilized; Aircraft X PIC transferred flight controls to SIC and went to the cabin to assess passenger injuries and damage. No reported passenger injuries and cabin appeared airworthy with some loose gear thrown about. Aircraft landed uneventfully at ZZZ1 62 minutes later. Postflight inspection and review of aircraft data files by company maintenance personnel revealed no apparent damage or exceedances and aircraft was returned to airworthy status. Contributing Factors: Aircraft X PIC poor assessment of convective turbulence threat using visual and onboard tools due to conditioning from previous successful weather deviations in general; and 20 minutes of successful deviations around this particular weather system prior to this encounter. Aircraft X PIC decision to accept unnecessary risk by not requesting further deviations to the west due to misplaced prioritization on mitigating perceived high ATC workload / task saturation versus ensuring safety of flight. Corrective Actions: Aircraft X PIC reset risk tolerance for operating in proximity to convective turbulence threat. Aircraft X PIC increase assertiveness in negotiating weather deviations with ATC when operating IFR in high traffic density environments.