Preheat oven to 400 degrees. Steam cauliflower until tender, about 10 minutes. Boil potatoes until just cooked through, about 15-20 minutes, depending on the size. Drain vegetables well and transfer to mixing bowl. Add chicken broth and milk and beat until smooth (cauliflower will be a little chunky so if you want a completely smooth texture, place cauliflower only in food processor and process until smooth - do not put potatoes in the food processor or they will turn gluey). Season to taste with salt and pepper. Transfer to baking dish and place cheese chunks on top of mashed cauliflower and potatoes, inserting a few pieces into the casserole dish.

Bake until cheese is melted, about 10 minutes.