A trick of ours is to use floury potatoes (i.e. Russet), which when cooked break down to thicken the curry slightly - or a portion of the potatoes could be blended and reincorporated. (101cookbooks.com)
was having trouble thickening so added about 3 - 4 TB plain Greek yogurt (one TB at a time) and about 2 TB tomato paste per some suggestions I read online about thickening curries. (simplyquinoa.com)
The coconut cream at the top of a can of coconut milk thickens this curry, so be careful not to shake the can before you open it. (vegetariantimes.com)