From memory, the knob just pulls off. That bit of coil showing is the retainer that secures the knob to the spindle. Pull that out with some needle-nose pliers and the broken bit of plastic will come with it. ECS Tuning has a replacement for a frankly outrageous US$69, though I suspect eBay will be somewhat cheaper (I just checked - it is, but not by much!). Part number is 8T0919070. There’s two versions - “A” has a long silvered skirt around the base of the knob, and “B” skips the skirt. Looks like yours is the B variant.