Place potatoes in a large pot and cover with 1 inch salted water. Boil potatoes until they are tender when pierced with a fork. Drain and mash. Set aside.
Meanwhile, melt butter in a large cast iron skillet over medium heat. Add celeriac and onions and cook until softened, about 3-5 minutes. Add cabbage, salt, pepper, coriander, garam masala, and chili powder. Stir well and cook for about 10 minutes, until cabbage is wilted and soft. Remove from heat.
Stir mashed potatoes into cast iron and mix until cabbage and potatoes are evenly distributed. If mixture is looking dry, drizzle olive oil on top and mix. Use a wooden spoon to pat down vegetables so they're even and flat, like a cake. Place cast iron back over medium high heat and cook, without touching, for about 10 minutes, until bottom side is browned.
Remove from heat and place a large platter or cutting board on top of cast iron and gently flip to turn cake upside down. Slice into large wedges and serve with sour cream on top.