Preheat the oven to 400 degrees F. Grease a 2.5 quart round baking dish or 9x13 rectangular baking dish and set aside. If using a round baking dish, the broth may not get as evenly absorbed as it would in a flatter, rectangular dish. This is not a problem when it comes to taste, but keep it in mind when choosing how you want to bake the casserole.
Place the sliced potatoes in a large pot and cover with salted water. Make sure you cover the potatoes by at least a couple of inches, so that there is room in the pot for the cauliflower to cook too. Bring the pot with the potatoes to a boil, add in the cauliflower florets, and cook for 8 to 10 minutes until the potatoes are tender. Drain and set aside to cool.
When the veggies are cool enough to handle, make a base layer of the veggies in the greased dish. Sprinkle with some of the cheese and chives, and season generously with salt and pepper. Repeat with another layer or two until all of the ingredients have been used. For a little extra flavor, consider adding pats of butter or drizzles of olive oil to each layer. Pour the vegetable broth into the dish.
Bake for 20 to 25 minutes until the cheese is bubbling and the veggies are golden brown. Remove from the oven and garnish with more fresh chives as desired.
I prefer to go light on the cheese, but feel free to add more cheese or even use two different kinds depending on how cheesy you want the dish to be.
Swap rosemary, parsley, or thyme for the chives if you have those on hand.
Add spoonfuls of broth from the bottom of the dish when serving for maximum flavor.
To make ahead of time, follow the instructions until putting the dish in the oven. Cover the uncooked dish with a lid, foil, or plastic wrap, and keep in the fridge for up to a day until you are ready to bake it.
Reheat leftovers in a baking dish at 300 degrees F for about 10 to 15 minutes until warmed through. Add additional broth if needed to prevent the veggies from drying out.