Potatoes, carrots and onions are the go-to veggies, but you have many additional options. I added green peas to add an extra pop of color and texture. I've also make it using 1-inch pieces of fresh haricot verts green beans and they added a similar layer of freshness and flavor. Sweet potatoes could be substituted for the white potatoes and would add a beautiful richness and sweetness to the curry. Other root vegetables like parsnips, rutabaga and celeriac would all work well in this dish.