To get the TV to upscale, you need to set both the Blu-Ray player and your Onkyo 876 to output 576p which is PAL's native resolution (or set the Onkyo to Through, which means it will just pass through whatever it gets from the Blu-Ray player without trying to upscale it). Since the TV is then receiving a standard def signal, it will then automatically upscale it to match its own 1080p resolution. To get the Onkyo to upscale, set it back to output 1080p, whilst leaving the Blu-Ray at 576p.
It's worth trying, but in general, the Blu-Ray players themselves tend to do a better job with upscaling than the TVs. If you do prefer the TV though, just remember you need to change the output on the Blu-Ray player back to 1080p when watching Blu-Rays!