Mix all the ingredients using a whisk.
Cover the vegetables thoroughly with the marinade (except the potatoes), making sure they're evenly coated and place them on a baking sheet that you can then use to take them to the grill. Marinade for 30 minutes.
Place the potatoes in a large cast iron skillet along with the remaining marinade, there should be quite a bit left. Mix well.
The potatoes take the longest: 30-40 minutes over medium heat and you have to stir every 10 minutes to get an even golden colour.
After the potatoes, the peppers take the longest, about 6-8 minutes per side. Then comes the zucchini, onion and spring onion which need 4 minutes per side. You want to see that they're charred but are still al dente.
Finish by adding the asparagus which only need 1-2 minutes per side. It's just a quick char and they're out.
Don't forget to season the veggies with more salt while they're on the grill!