Delicious! It's been a while since I had real haggis, but I think this is a fair approximation, and all with veggies! It's quite easy (especially compared to traditional recipes) - no tracking down strange bits of offal, which are hard/impossible to find in the States.
I was confused about the "mixed spice" and didn't even think of the pie spices other people have named, so I just ended up using a basil/oregano/sage mix and it was fine. I also subbed pecans for hazelnuts, since I had them on hand. One tough spot was that the mixture was too dry already when starting in on the 20 minute simmer, so I added water along the way to keep it from burning to the bottom of the pan and drying out. It also didn't set up into a real firm loaf, but served as a sort of hash was just fine. What matters is that its delicious. Cheers!